Multifocal contact lens

ABSTRACT

There is provided a multifocal contact lens having a lens curve formed by altermately arranging a plurality of curved surfaces for far vision and a plurality of curved surfaces for near vision in the form of concentric zones, and a die for forming the contact lens, and a method for producing the contact lens. Each of the curved surfaces (F1, F2, . . . ) for far vision of the lens curve (2) has a center (O F1 , O F2 , . . . ) of curvature on an optical axis and a radius (R F1 , R F2 , . . . ) of curvature, which is set so that a ray being incident on the corresponding curved surface and being parallel to the optical axis forms an image at a location near a single focal point (F F ) for far vision on the optical axis, and each of the curved surfaces (N1, N2, . . . ) for near vision of the lens curve (2) has a center (O N1 , O N2 , . . . ) of curvature on the optical axis and a radius (R N1 , R N2 , . . . ) of curvature, which is set so that a ray being incident on the corresponding curved surface and being parallel to the optical axis forms an image at a location near a single focal point (F N ) for near vision on the optical axis.

BACKGROUND OF THE INVENTION

1. Field of The Invention

The present invention relates generally to a multifocal contact lens, amold for forming the same, and a method for producing the same. Morespecifically, the invention relates to a multifocal contact lens with alens curve formed by alternately arranging a plurality of curvedsurfaces for far vision and a plurality of curved surface for nearvision in the form of concentric zones, a die for forming the same, amethod for manufacturing the die, and a method for producing themultifocal contact lens.

2. Description of The Prior Art

A multifocal contact lens, wherein a plurality of portions for farvision and a plurality of portions for near vision are altermatelyarranged in the form of concentric zones, has been proposed in, e.g.,Japanese Patent Laid-Open No. 59-146020. When a user wears such acontact lens, the user can consciously choose one of far and near rangeswhich can be simultaneously viewed by the user. This contact lens isuseful since the user can naturally and smoothly choose one of the farand near ranges.

As shown in FIG. 8, such a contact lens 1 has a front curve 2 formed byaltermately arranging a plurality of curved surfaces F1, F2, . . . forfar vision and a plurality of curved surfaces N1, N2, . . . for nearvision in the form of concentric zones, and a base curve 3 having ashape corresponding to the curved surface of the user's cornea.

In conventional contact lenses, the curved surfaces F1, F2, . . . forfar vision and the curved surfaces N1, N2, . . . for near vision areformed on the front curve 2 as follows.

It is assumed that the radius of curvature of the curved surface for farvision is R_(F) and the radius of curvature of the curved surface fornear vision is R_(N). First, a circle having a radius R_(F) from a pointP on an optical axis corresponding to Z-axis is described to derive anintersection point with the optical axis. It is assumed that thisintersection point is a center O_(F1) of curvature of the curved surfaceF1 for far vision. Then, a circle having the radius R_(F) is describedabout the center O_(F1) of curvature to derive an intersection pointP_(F1) with a straight line l_(F1) which defines a predetermined zonewidth of the curved surface F1 for far vision and which is parallel tothe optical axis. Then, a circle having the radius R_(N) is describedabout the point P_(F1) to derive an intersection point with the opticalaxis. In is assumed that this intersection point is a center O_(N1) ofcurvature of the curved surface N1 for near vision. Then, a circlehaving the radius R_(N) is described about the center O_(N1) ofcurvature to derive an intersection point P_(N1) with a straight linel_(N1) defining a predetermined zone width of the curved surface N1 fornear vision.

Similarly, a circle having the radius R_(F) is described about the pointP_(N1) to derive an intersection point with the optical axis. It isassumed that this intersection point is a center O_(F2) of curvature ofthe curved surface F2 for far vision. Then, a circle having the radiusR_(F) is described about the center O_(F2) of curvature to derive anintersection point P_(F2) with a straight line l_(F2) which defines apredetermined zone width of the curved surface F2 for far vision andwhich is parallel to the optical axis. Then, a circle having the radiusR_(N) is described about the point P_(F2) to derive an intersectionpoint with the optical axis. It is assumed that this intersection pointis a center O_(N2) of curvature of the curved surface N2 for nearvision.

The centers O_(F1), O_(F2), . . . of curvature of the curved surfacesF1, F2, . . . for far vision and the centers O_(N1), O_(N2), . . . ofcurvature of the curved surfaces N1, N2, . . . for near vision thusobtained are shown in FIG. 7.

As can be clearly seen from FIG. 7, the centers O_(F1), O_(F2), . . . ofcurvature of the curved surfaces F1, F2, . . . for far vision aredistributed so as to be sequentially shifted in a direction of Z-axis,i.e., from the base curve 3 toward the front curve 2.

As a result, as shown in FIG. 6, rays, which are incident on the curvedsurfaces F1, F2, . . . for far vision and which are parallel to theoptical axis, form images at the respective focal points F_(F1), F_(F2),. . . of the curved surfaces F1, F2, . . . for far vision, so that theimage formation is not carried out at a point. Similarly, rays, whichare parallel to rays being incident on the curved surfaces N1, N2, . . .for near vision, form images at the respective focal points F_(N1),F_(N2), . . . of the curved surfaces N1, N2, . . . for near vision, sothat the image formation is not carried out at a point. That is, thereis a problem in that the spherical aberration of the conventionalcontact lens is too great to obtain a clear image.

Furthermore, as shown in FIG. 6, since the spherical aberration of theperiphery of the contact lens 1 is great, the order of arrangement ofthe focal points F_(F1), F_(F2), . . . for far vision is the reverse ofthe order of arrangement of the centers O_(F1), O_(F2), . . . ofcurvature of the curved surfaces F1, F2, . . . for far vision.

In order to eliminate such a problem, a multifocal contact lens has beenproposed by the applicant of the instant application as shown in FIG. 5(e.g., Japanese Patent Application No. 5-508019 (InternationalPublication No. 93/14434)).

As shown in FIG. 5, a contact lens 1 has a front curve 2 formed byaltermately arranging curved surfaces F1, F2, . . . for far vision andcurved surfaces N1, N2, . . . for near vision in the form of concentriczones, and a base curve 3. It is assumed herein that the optical axis ofthe contact lens 1 is Z-axis and the direction of the Z-axis is from thefront curve 2 toward the base curve 3. It is also assumed that X-axispasses through the vertex P of the contact lens 1 and is perpendicularto the Z-axis.

The shape of the curved surface of the base curve 3 is chosen so as tocorrespond to the shape of the curved surface of the user's cornea. Onthe basis of the values for the chosen shape of the curved surface ofthe base curve 3, the radius R_(F) of curvature of the curved surfacesF1, F2, . . . for far vision and the radius R_(N) of curvature of thecurved surfaces N1, N2, . . . for near vision, which are required toobtain desired powers of the portions for far vision and desired addedpowers of the portions for near vision, are defined.

The centers O_(F1), O_(F2), . . . and O_(N1), O_(N2), . . . of curvatureof the curved surfaces F1, F2, . . . for far vision of the front curve 2and the curved surface N1, N2, . . . for near vision of the front curve2 are derived as follows.

As shown in FIG. 4, an intersection point of parallel rays beingincident on the curved surface F1 for far vision and outgoing the basecurve 3, with the optical axis is derived, and this intersection pointis defined as a focal point F_(F) for far vision. In addition, anintersection point of parallel rays being incident on the curved surfaceN1 for near vision and outgoing the base curve 3, with the optical axisis derived, and this intersection point is defined as a focal pointF_(N) for near vision.

First, it is assumed that the position on the optic axis, which is apartfrom the vertex P by the radius R_(F) of curvature of the portion forfar vision, is the center O_(F1) of curvature of the curved surface F1for far vision. Then, a circle having the radius R_(F) is describedabout the center O_(F1) of curvature to derive an intersection pointP_(F1) with a straight line l_(F1), which defines a predetermined zonewidth of the curved surface F1 for far vision and which is parallel tothe optical axis. Then, a circle having the radius R_(N) is describedabout the point P_(F1) to derive an intersectional point with theoptical axis. It is assumed that this intersectional axis is the centerO_(N1) of curvature of the curved surface N1 for near vision. Thesesteps are the same as those of the conventional method shown in FIG. 8.

Then, as shown in FIG. 5, the center O_(F2) of curvature of the curvedsurface F2 for far vision is derived as follows. That is, a circlehaving the radius R_(N) is described about the center O_(N1) ofcurvature to derive an intersection point P_(N1) with a straight linel_(N1), which defines a predetermined zone width of the curved surfaceN1 for near vision and which is parallel to the optical axis. Then, acircle having the radius R_(F) is described about the point P_(N1) toderive an intersection point with the optical axis. This intersectionpoint is defined as a proposed point for the point O_(F2) which is to bethe center of curvature of the curved surface F2 for far vision.

Then, the proposed point for the center O_(F2) of curvature is used as astart point to derive a rightful center O_(F2) of curvature near theproposed point using the ray tracing method. Specifically, the centerO_(F2) of curvature is derived using the ray tracing method as follows.

First, a circle having the radius R_(F) is described about the proposedpoint for the center O_(F2) of curvature to derive an intersection pointof this circle with a straight line l_(F2). It is assumed that thisintersection point is a proposed point for the point P_(F2) and that thecurved surface extending from the point P_(N1) to the point P_(F2) is aproposed curved surface for the curved surface F2 for far vision.

Then, parallel rays are incident on the proposed curved surface for thecurved surface F2 for far vision in the zone between the straight linesl_(N1) and l_(F2) to derive an intersection of the parallel raysoutgoing the base curve 3 with the optical axis.

In a case where the intersection of the incident parallel rays with theoptical axis is shifted from the focal point F_(F) for far visiondefined in FIG. 4 in a negative direction of Z-axis, the proposed curvedsurface for the curved surface F2 for far vision is rotated slightlycounterclockwise about the point P_(N1) in FIG. 5.

On the other hand, in a case where the intersection of the incidentparallel rays with the optical axis is shifted from the focal pointF_(F) for far vision in a positive direction of Z-axis, the proposedcurved surface for the curved surface F2 for far vision is rotatedclockwise about the point P_(N1).

Thus, the rotating direction and amount of the proposed curved surfacefor the curved surface F2 for far vision rotated about the point P_(N1)are derived and the rotating position is decided so that the parallelrays being incident on the front curve 2 pass through the focal pointF_(F) for far vision. The curved surface, which is obtained by rotatingthe proposed curved surface for the curved surface F2 for far visionabout the point P_(N1) to the decided rotating position, is a curvedsurface F2 for far vision to be derived, and the decided center ofcurvature of the curved surface F2 for far vision is a center O_(F2) ofcurvature to be derived.

Then, a center O_(N2) of curvature of the curved surface N2 for nearvision is derived as follows. That is, a circle having the radius R_(F)is described about the decided center O_(F2) of curvature to derive anintersection point with a straight line l_(F2), which defines apredetermined zone width of the curved surface F2 for far vision andwhich is parallel to the optical axis, and this intersection point isdefined as a point P_(F2). Then, a circle having the radius R_(N) isdescribed about the point P_(F2) to derive an intersection point withthe optical axis, and this intersection point is assumed as a proposedpoint for the center O_(N2) of curvature of the curved surface N2 fornear vision.

Then, the proposed point for the center O_(N2) of curvature is used as astart point to derive a rightful center O_(N2) of curvature near theproposed point using the ray tracing method. In order to derive therightful center O_(N2), a circle having the radius R_(N) is describedabout the proposed point for the center O_(N2) of curvature to derive anintersection point of this circle with the straight line l_(N2). Thisintersection point P_(N2) is used as a proposed point to define theshape of a proposed curved surface for the curved surface N2 for nearvision extending from the point P_(F2) to the point P_(N2).

Then, parallel rays in the range between the straight lines l_(F2) andl_(N2) are incident on the proposed curved surface for the curvedsurface for near vision to derive intersections of the parallel rayswith the optical axis after outgoing the base curve 3.

In a case where the intersections of the incident parallel rays with theoptical axis are shifted from the focal point F_(N) for near visionshown in FIG. 4 in a negative direction of Z-axis, the proposed curvedsurface for the curved surface N2 for near vision is rotated slightlycounterclockwise about the point P_(F2) in FIG. 5.

On the other hand, in a case where the intersections of the incidentparallel rays with the optical axis are shifted from the focal pointF_(N) for near vision in a positive direction of Z-axis, the proposedcurved surface for the curved surface N2 for near vision is rotatedclockwise about the point P_(F2).

Thus, the direction and amount of the proposed curved surface for thecurved surface N2 for near vision rotating about the point P_(F2) isderived so that the parallel rays being incident on the front curve 2pass through the focal point F_(N) for near vision, and the rotatingposition is decided. The curved surface, which is obtained by rotatingthe proposed curved surface for the curved surface N2 for near visionabout the point P_(F2) to the decided rotating position, is a curvedsurface N2 for near vision to be derived, and the center of curvature ofthe decided curved surface N2 for near vision is a center O_(N2) ofcurvature to be derived.

Similarly, other centers O_(F3), O_(F4), . . . and O_(N3), O_(N4), . . .of curvature are derived, so that the contact lens 1 shown in FIG. 5 isobtained.

However, in the case of the contact lens shown in FIG. 5, the sphericalaberration may be sufficiently corrected for the following reasons.

That is, as shown in FIG. 5, although the center O_(F1) of curvature ofthe curved surface F1 for far vision and the center O_(N1) of curvatureof the curved surface N1 for near vision are located on the opticalaxis, the centers O_(F2), O_(F3), . . . of curvature of the curvedsurfaces F2, F3, . . . for far vision and the centers O_(N2), O_(N3), .. . of curvature of the curved surface N2, N3, . . . for near vision arenot located on the optical axis to be apart from the optical axis in adirection of X-axis. In addition, the ray tracing method is appliedunder the restriction that the curved surfaces F1, F2, F3, . . . for farvision must have the same radius R_(N) of curvature and the curvedsurface N1, N2, N3, . . . for near vision must have the same radiusR_(N) of curvature When the ray tracing method is applied, the degree offreedom of design is decreased, so that the spherical aberration mayremain as follows.

For example, with respect to the whole predetermined zone widths of thecurved surface F2 for far vision and the curved surface N2 for nearvision, there is considered parallel rays being incident in parallel tothe optical axis and outgoing the base curve 3 to travel to theintersection with the optical axis. As shown in FIG. 4, these parallelrays converge before the optical axis after outgoing the base curve 3,and then, they are divergent rays. Therefore, no image is formed at apoint on the optical axis defined by the line extending from the vertexP of the contact lens to the center O_(F1) of curvature of the curvedsurface F1 for far vision, so that spherical aberrations remain in therespective spherical surfaces of the curved surface F2 for far visionand the curved surface N2 for near vision.

More specifically, the parallel rays being incident on the curvedsurface F2 for far vision at a predetermined zone width, outgo the basecurve 3 to converge at the respective points on a ring, which isdescribed about the Z-axis so as to have a radius corresponding to thedistance between the Z-axis and the point O_(F2). Thereafter, the raysbecome divergent rays, so that an image is formed on the Z-axis in theform of a line, not a point. In addition, the parallel rays beingincident on the curved surface N2 for near vision at a predeterminedzone width, outgo the base curve 3 to converge at the respective pointson a ring, which is described about the Z-axis so as to have a radiuscorresponding to the distance between the Z-axis and the point O_(N2).Thereafter, the rays become divergent rays, so that an image is formedon the Z-axis in the form of a line, not a point. Thus, the parallelrays being incident on the respective curved surface do not form imagesat the focal point F_(F) for far vision and the focal point F_(N) fornear vision on the curved surface defined above, so that sphericalaberrations remain in the respective curved surfaces. This is the samewith respect to the curved surfaces F2, F3, . . . for far vision and thecurved surfaces N2, N3, . . . for near vision determined by the raytracing method.

In addition, the dimension of the spherical aberration is under theinfluence of the absolute quantity of variation of the respectivecenters O_(F2), O_(F3), . . . and O_(N2), O_(N3), . . . of curvature ina direction of X-axis when the shapes of the curved surfaces F2, F3, . .. for far vision and the curved surfaces N2, N3, . . . for near visionare determined by the ray tracing method.

More specifically, when the shapes of the curved surfaces F2, F3, . . .for far vision and the curved surfaces N2, N3, . . . for near vision aredecided by the ray tracing method, as the distances between therespective centers O_(F2), O_(F3), . . . and O_(N2), O_(N3), . . . ofcurvature and the optical axis in a direction of the X-axis isincreased, the spherical aberration is increased.

Thus, in the case of the contact lens shown in FIG. 5, although thecenter O_(F1) of curvature and the center O_(N1) of curvature arelocated on the optical axis, the centers O_(F2), O_(F3), . . . ofcurvature of the other curved surfaces F2, F3, . . . for far vision andthe centers O_(N2), O_(N3), . . . of curvature of the other curvedsurfaces N2, N3, . . . for near vision are not located on the opticalaxis. In addition, the ray tracing method is applied under therestriction that the curved surfaces F1, F2, F3, . . . for far visionhave the same radius R_(F) of curvature and the curved surfaces N1, N2,N3, . . . for near vision have the same radius R_(N) of curvature.Therefore, there are problems in that the degree of freedom of design isnot sufficient and that spherical aberration may remain.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to eliminate theaforementioned problems and to provide a multifocal contact lens, whichhas a lens curve formed by altermately arranging a plurality of curvedsurfaces for far vision and near vision in concentric zones and whichcan suitably remove spherical aberrations to obtain a clear visualacuity, and a mold for forming the multifocal contact lens.

It is another object of the present invention to provide a multifocalcontact lens, which can obtain substantially the same performance asthat of a single focal lens by putting it to proper use in accordancewith indoor and outdoor uses and which has a bifocal function for alongsighted and shortsighted person.

It is further object of the present invention to provide a method forproducing a multifocal contact lens, by which the boundary between theadjacent curved surfaces for far vision and near vision of the lens canbe uniformly polished.

In order to accomplish the aforementioned and other objects, accordingto one aspect of the present invention, there is provides a multifocalcontact lens having a lens curve formed by altermately arranging aplurality of curved surfaces for far vision and a plurality of curvedsurfaces for near vision in the form of concentric zones, wherein eachof the plurality of curved surfaces for far vision of the lens curve hasa center of curvature on an optical axis and a radius of curvature,which is set so that a ray being incident on the corresponding curvedsurface and being parallel to the optical axis forms an image at alocation near a single focal point for far vision on the optical axis,and wherein each of the plurality of curved surfaces for near vision ofthe lens curve has a center of curvature on the optical axis and aradius of curvature, which is set so that a ray being incident on thecorresponding curved surface and being parallel to the optical axisforms an image at a location near a single focal point for near visionon the optical axis.

According to another aspect of the present invention, there is provideda multifocal contact lens having a lens curve formed by alternatelyarranging a plurality of curved surfaces for far vision and a pluralityof curved surfaces for near vision in the form of concentric zones,wherein each of the plurality of curved surfaces for far vision of thelens curve has a center of curvature on an optical axis and a radius ofcurvature, which is set so that a predetermined principal ray beingincident on the corresponding curved surface and being parallel to theoptical axis passes through a location near a single focal point for farvision on the optical axis, and wherein each of the plurality of curvedsurfaces for near vision of the lens curve has a center of curvature onthe optical axis and a radius of curvature, which is set so that apredetermined principal ray being incident on the corresponding curvedsurface and being parallel to the optical axis passes through a locationnear a single focal point for near vision on the optical axis.

As the predetermined principal ray, a ray passing through apredetermined location in a zone width of each of the curved surfacesfor far vision and near vision may be selected with respect to all thecurved surfaces for far vision and near vision, which include the curvedsurface for far vision containing the optical axis and the curvedsurface for near vision containing the optical axis. The predeterminedlocation in the zone width of each of all the curved surfaces may be thecenter in the zone width of each of all the curved surfaces.Alternatively, as the predetermined principal ray, a ray, whichcorresponds to the optical axis with respect to a curved surface for farvision containing the optical axis and a curved surface for near visioncontaining the optical axis and which passes through a predeterminedlocation in a zone width of each of other curved surfaces for far visionand near vision may be selected with respect to the other curvedsurfaces for far vision and near vision. Each of the curved surfaces forfar vision may have a radius of curvature which is different from thoseof other curved surfaces for far vision, and each of the curved surfacesfor near vision may have a radius of curvature which is different fromthose of other curved surfaces for near vision. The lens curve may be afront curve. The zone width of each of the curved surfaces for farvision may vary in accordance with the distance between each of thecurved surfaces for far vision and the optical axis, and the zone widthof each of the curved surfaces for near vision may vary in accordancewith the distance between each of the curved surfaces for near visionand the optical axis. Specifically, the zone width of each of the curvedsurfaces for far vision may increase in accordance with the distancebetween each of the curved surfaces for far vision and the optical axis,and the zone width of each of the curved surfaces for near vision mayincrease in accordance with the distance between each of the curvedsurfaces for near vision and the optical axis. Alternatively, the zonewidth of each of the curved surfaces for far vision may decrease inaccordance with the distance between each of the curved surfaces for farvision and the optical axis, and the zone width of each of the curvedsurfaces for near vision may decrease in accordance with the distancebetween each of the curved surfaces for near vision and the opticalaxis. That is, the zone width of each of the curved surfaces for farvision may decrease or increase in accordance with the distance betweeneach of the curved surfaces for far vision and the optical axis, and thezone width of each of the curved surfaces for near vision may decreaseor increase in accordance with the distance between each of the curvedsurfaces for near vision and the optical axis. Alternatively, the zonewidth of each of the curved surfaces for far vision may be substantiallythe same as those of other curved surfaces for far vision, and the zonewidth of each of the curved surfaces for near vision may besubstantially the same as those of other curved surfaces for nearvision. In addition, an energy ratio of the curved surface for farvision to the curved surface for near vision may be set in accordancewith indoor or outdoor use. The energy ratio may be an area ratio of thecurved surface for far vision to the curved surface for near vision.Alternatively, the energy ratio may be a ratio of amount of transmittedlight.

According to another aspect of the present invention, there is provideda mold for forming a multifocal contact lens having a lens curve formedby altermately arranging a plurality of curved surfaces for far visionand a plurality of curved surfaces for near vision in the form ofconcentric zones, wherein each of the plurality of curved surfaces forfar vision of the lens curve has a center of curvature on an opticalaxis and a radius of curvature, which is set so that a ray beingincident on the corresponding curved surface and being parallel to theoptical axis forms an image at a location near a single focal point forfar vision on the optical axis, and wherein each of the plurality ofcurved surfaces for near vision of the lens curve has a center ofcurvature on the optical axis and a radius of curvature, which is set sothat a ray being incident on the corresponding curved surface and beingparallel to the optical axis forms an image at a location near a singlefocal point for near vision on the optical axis.

According to another aspect of the present invention, there is provideda mold for forming a multifocal contact lens having a lens curve formedby altermately arranging a plurality of curved surfaces for far visionand a plurality of curved surfaces for near vision in the form ofconcentric zones, wherein each of the plurality of curved surfaces forfar vision of the lens curve has a center of curvature on an opticalaxis and a radius of curvature, which is set so that a predeterminedprincipal ray being incident on the corresponding curved surface andbeing parallel to the optical axis passes through a location near asingle focal point for far vision on the optical axis, and wherein eachof the plurality of curved surfaces for near vision of the lens curvehas a center of curvature on the optical axis and a radius of curvature,which is set so that a predetermined principal ray being incident on thecorresponding curved surface and being parallel to the optical axispasses through a location near a single focal point for near vision onthe optical axis.

According to another aspect of the present invention, there is provideda method for producing a multifocal contact lens having a lens curveformed by altermately arranging a plurality of curved surfaces for farvision and a plurality of curved surfaces for near vision in the form ofconcentric zones, the method comprising the steps of: determining afocal point for far vision and a focal point for near vision on anoptical axis; sequentially setting a proposal for a center of curvatureand a proposal for a radius of curvature, which define each of thecurved surfaces for far vision and near vision, with respect to each ofthe curved surfaces for far vision and near vision in the order of fromthe curved surface for far vision or near vision nearest the opticalaxis toward the curved surface for near vision or far vision apart fromthe optical axis; sequentially changing the proposal for the center ofcurvature and the proposal for the radius of curvature to carry out theray tracing so that a predetermined principal ray being parallel to theoptical axis passes through the focal point for far vision or the focalpoint for near vision; and deciding the center of curvature of each ofthe curved surfaces on the optical axis, and the radius of curvature ofeach of the curved surfaces.

The method for producing a multifocal contact lens may further comprisethe steps of: defining a focal point for far vision and a focal pointfor near vision on an optical axis; setting a proposal for a center ofcurvature of a first portion for far vision and a proposal for a radiusof curvature of the first portion for far vision, the center ofcurvature and the radius of curvature of the first portion for farvision defining a first curved surface for far vision containing theoptical axis; deriving an intersection point of a circle, which isdescribed about the proposal for the center of curvature of the firstportion for far vision so as to have a radius being the same as theproposal for the radius of curvature of the first portion for farvision, with a straight line of the first portion for far vision, whichis parallel to the optical axis defining a zone width of the firstcurved surface for far vision; sequentially changing the proposal forthe center of curvature of the first portion for far vision and theproposal for the radius of curvature of the first portion for far visionto carry out the ray tracing so that a predetermined principal ray,which is incident on the proposal for the first curved surface for farvision and which is parallel to the optical axis, passes through thefocal point for far vision, to decide the center of curvature of thefirst portion for far vision and the radius of curvature of the firstportion for far vision; deriving an intersection point of a circle,which is described about the decided center of curvature of the firstportion for far vision so as to have the radius of curvature of thefirst portion for far vision, with the straight line of the firstportion for far vision as an intersection point of the first portion forfar vision, and deciding a curved surface extending from a vertex of thelens curve to the intersection point of the first portion for far visionas the first curved surface for far vision; setting a proposal for aradius of curvature of a first portion for near vision defining a firstcurved surface for near vision outwards adjacent to the first curvedsurface for far vision, and deriving an intersection point of a circle,which is described about the intersection point of the first portion forfar vision so as to have a radius being the same as the proposal for theradius of curvature of the first portion for far vision, with theoptical axis as a proposal for a center of curvature of the firstportion for near vision; deriving an intersection point of a circle,which is described about the proposal for the center of curvature of thefirst portion for near vision so as to have a radius being the same asthe proposal for the radius of curvature of the first portion for nearvision, with a straight line of the first portion for near vision, whichdefines a zone width of the first curved surface for near vision andwhich is parallel to the optical axis, to derive a proposal for thefirst curved surface for near vision; sequentially changing the proposalfor the center of curvature of the first portion for near vision and theproposal for the radius of curvature of the first portion for nearvision, to carry out the ray tracing so that a predetermined principalray, which is incident on the proposal for the curved surface of thefirst portion for near vision and which is parallel to the optical axis,to decide the center of curvature of the first portion for near visionand the radius of curvature of the first portion for near vision;deriving an intersection point of a circle, which is described about thedecided center of curvature of the first portion for near vision so asto have a radius being the same as the radius of curvature of the firstportion for near vision, with the straight line of the first portion fornear vision as an intersection point of the first portion for nearvision, and deciding a curved surface extending from the intersectionpoint of the first portion for far vision to the intersection point ofthe first portion for near vision as the first curved surface for nearvision; setting a proposal for a radius of curvature of a second portionfor far vision defining a curved surface of a second portion for farvision outsides adjacent to the first curved surface for near vision,and deriving an intersection point of a circle, which is described aboutthe intersection point of the first portion for near vision so as tohave a radius being the same as the proposal for the radius of curvatureof the second portion for far vision, with the optical axis as aproposal for a center of curvature of the second portion for far vision;deriving an intersection point of a circle, which is described about theproposal for the center of curvature of the second portion for farvision so as to have a radius being the same as the proposal for theradius of curvature of the second portion for far vision, with astraight line of the second portion for far vision, which defines a zonewidth of the second curved surface for far vision and which is parallelto the optical axis; sequentially changing the proposal for the centerof curvature of the second portion for far vision and the proposal forthe radius of curvature of the second portion for far vision, to carryout the ray tracing so that a predetermined principal ray, which isincident on the proposal for the second curved surface for far visionand which is parallel to the optical axis, passes through the focalpoint for far vision, to decide the center of curvature of the secondportion for far vision and the radius of curvature of the second portionfor far vision; deriving an intersection point of a circle, which isdescribed about the decided center of curvature of the second portionfor far vision so as to have a radius being the same radius of curvatureof the second portion for far vision, with the straight line of thesecond portion for far vision as an intersection portion of the secondportion for far vision, and deciding a curved surface extending from theintersection point of the first portion for near vision to theintersection point of the second portion for far vision as the secondcurved surface for far vision; setting a proposal for a radius ofcurvature of a second portion for near vision defining a curved surfaceof a second portion for near vision outwards adjacent to the secondcurved surface for far vision, and deciding an intersection point of acircle, which is described about the intersection point of the secondportion for far vision so as to have a radius being the same as theproposal for the radius of curvature of the second portion for nearvision, with the optical axis as a proposal for a center of curvature ofthe second portion for near vision; deriving an intersection point of acircle, which is described about the proposal for the center ofcurvature of the second portion for near vision so as to have a radiusbeing the same as the proposal for the radius of curvature of the secondportion for near vision, with a straight line of a second portion fornear vision, which defines a zone width of the second curved surface fornear vision and which is parallel to the optical axis, to derive aproposal for the second curved surface for near vision; sequentiallychanging the proposal for the center of curvature of the second portionfor near vision and the proposal for the radius of curvature of thesecond portion for near vision, to carry out the ray tracing so that apredetermined principal ray, which is incident on the proposal for thesecond curved surface for near vision and which is parallel to theoptical axis, passes through the focal point for near vision, to decidethe center of curvature of the second portion for near vision and theradius of curvature of the second portion for near vision; and derivingan intersection point of a circle, which is described about the decidedcenter of curvature of the second portion for near vision so as to havea radius being the same as the radius of curvature of the second portionfor near vision, with the straight line of the second portion for nearvision as an intersection point of the second portion for near vision,and deciding a curved surface extending from the intersection point ofthe second portion for far vision to the intersection point of thesecond portion for near vision as the second curved surface for nearvision.

Alternatively the method for producing a multifocal contact lens mayfurther comprise the steps of: defining a focal point for far vision anda focal point for near vision on an optical axis; setting a proposal fora center of curvature of a first portion for near vision and a proposalfor a radius of curvature of a first portion for near vision, the centerof curvature and the radius of curvature of the first portion for nearvision defining a first curved surface for near vision containing theoptical axis; deriving an intersection point of a circle, which isdescribed about the proposal for the center of curvature of the firstportion for near vision so as to have a radius being the same as theproposal for the radius of curvature of the first portion for nearvision, with a straight line of the first portion for near vision, whichis parallel to the optical axis defining a zone width of the firstcurved surface for near vision; sequentially changing the proposal forthe center of curvature of the first portion for near vision and theproposal for the radius of curvature of the first portion for nearvision to carry out the ray tracing so that a predetermined principalray, which is incident on the proposal for the first curved surface fornear vision and which is parallel to the optical axis, passes throughthe focal point for near vision, to decide the center of curvature ofthe first portion for near vision and the radius of curvature of thefirst portion for near vision; deriving an intersection point of acircle, which is described about the decided center of curvature of thefirst portion for near vision so as to have the radius of curvature ofthe first portion for near vision, with the straight line of the firstportion for near vision as an intersection point of the first portionfor near vision, and deciding a curved surface extending a vertex of thelens curve to the intersection point of the first portion for nearvision as the first curved surface for near vision; setting a proposalfor a radius of curvature of a first portion for far vision defining afirst curved surface for far vision outwards adjacent to the firstcurved surface for near vision, and deriving an intersection point of acircle, which is described about the intersection point of the firstportion for near vision so as to have a radius being the same as theproposal for the radius of curvature of the first portion for nearvision, with the optical axis as a proposal for a center of curvature ofthe first portion for far vision; deriving an intersection point of acircle, which is described about the proposal for the center ofcurvature of the first portion for far vision so as to have a radiusbeing the same as the proposal for the radius of curvature of the firstportion for far vision, with a straight line of the first portion forfar vision, which defines a zone width of the first curved surface forfar vision and which is parallel to the optical axis, to derive aproposal for the first curved surface for far vision; sequentiallychanging the proposal for the center of curvature of the first portionfor far vision and the proposal for the radius of curvature of the firstportion for far vision, to carry out the ray tracing so that apredetermined principal ray, which is incident on the proposal for thecurved surface of the first portion for far vision and which is parallelto the optical axis, to decide the center of curvature of the firstportion for far vision and the radius of curvature of the first portionfor far vision; deriving an intersection point of a circle, which isdescribed about the decided center of curvature of the first portion forfar vision so as to have a radius being the same as the radius ofcurvature of the first portion for far vision, with the straight line ofthe first portion for far vision as an intersection point of the firstportion for far vision, and deciding a curved surface extending from theintersection point of the first portion for near vision to theintersection point of the first portion for far vision as the firstcurved surface for far vision; setting a proposal for a radius ofcurvature of a second portion for near vision defining a curved surfaceof a second portion for near vision outsides adjacent to the firstcurved surface for far vision, and deriving an intersection point of acircle, which is described about the intersection point of the firstportion for far vision so as to have a radius being the same as theproposal for the radius of curvature of the second portion for nearvision, with the optical axis as a proposal for a center of curvature ofthe second portion for near vision; deriving an intersection point of acircle, which is described about the proposal for the center ofcurvature of the second portion for near vision so as to have a radiusbeing the same as the proposal for the radius of curvature of the secondportion for near vision, with a straight line of the second portion fornear vision, which defines a zone width of the second curved surface fornear vision and which is parallel to the optical axis; sequentiallychanging the proposal for the center of curvature of the second portionfor near vision and the proposal for the radius of curvature of thesecond portion for near vision, to carry out the ray tracing so that apredetermined principal ray, which is incident on the proposal for thesecond curved surface for near vision and which is parallel to theoptical axis, passes through the focal point for near vision, to decidethe center of curvature of the second portion for near vision and theradius of curvature of the second portion for near vision; deriving anintersection point of a circle, which is described about the decidedcenter of curvature of the second portion for near vision so as to havea radius being the same radius of curvature of the second portion fornear vision, with the straight line of the second portion for nearvision as an intersection portion of the second portion for near vision,and deciding a curved surface extending from the intersection point ofthe first portion for far vision to the intersection point of the secondportion for near vision as the second curved surface for near vision;setting a proposal for a radius of curvature of a second portion for farvision defining a second curved surface for far vision outwards adjacentto the second curved surface for near vision, and deciding anintersection point of a circle, which is described about theintersection point of the second portion for near vision so as to have aradius being the same as the proposal for the radius of curvature of thesecond portion for far vision, with the optical axis as a proposal for acenter of curvature of the second portion for far vision; deriving anintersection point of a circle, which is described about the proposalfor the center of curvature of the second portion for far vision so asto have a radius being the same as the proposal for the radius ofcurvature of the second portion for far vision, with a straight line ofthe second portion for far vision, which defines a zone width of thesecond curved surface for far vision and which is parallel to theoptical axis, to derive a proposal for the second curved surface for farvision; sequentially changing the proposal for the center of curvatureof the second portion for far vision and the proposal for the radius ofcurvature of the second portion for far vision, to carry out the raytracing so that a predetermined principal ray, which is incident on theproposal for the second curved surface for far vision and which isparallel to the optical axis, passes through the focal point for farvision, to decide the center of curvature of the second portion for farvision and the radius of curvature of the second portion for far vision;and deriving an intersection point of a circle, which is described aboutthe decided center of curvature of the second portion for far vision soas to have a radius being the same as the radius of curvature of thesecond portion for far vision, with the straight line of the secondportion for far vision as an intersection point of the second portionfor far vision, and deciding a curved surface extending from theintersection point of the second portion for near vision to theintersection point of the second portion for far vision as the secondcurved surface for far vision.

In the aforementioned method for a multifocal contact lens, thepredetermined principal ray may be a ray passing through a predeterminedlocation in a zone width of each of the curved surfaces for far visionand near vision with respect to all the curved surfaces for far visionand near vision, which include the curved surface for far visioncontaining the optical axis and the curved surface for near visioncontaining the optical axis. The predetermined location in the zonewidth of each of all the curved surfaces may be the center in the zonewidth of each of all the curved surfaces. Alternatively, thepredetermined principal ray may be a ray, which corresponds to theoptical axis with respect to a curved surface for far vision containingthe optical axis and a curved surface for near vision containing theoptical axis and which passes through a predetermined location in a zonewidth of each of curved surfaces with respect to other curved surfacesfor far vision and near vision. Each of the curved surfaces for farvision may have a radius of curvature which is different from those ofother curved surfaces for far vision, and each of the curved surfacesfor near vision may have a radius of curvature which is different fromthose of other curved surfaces for near vision. The lens curve may be afront curve.

According to further aspect of the present invention, there is provideda method for producing a multifocal contact lens having a lens curveformed by altermately arranging a plurality of curved surfaces for farvision and a plurality of curved surfaces for near vision in the form ofconcentric zones, wherein each of the plurality of curved surfaces forfar vision of the lens curve has a center of curvature on an opticalaxis and a radius of curvature, which is set so that a ray beingincident on the corresponding curved surface and being parallel to theoptical axis forms an image at a location near a single focal point forfar vision on the optical axis, and wherein each of the plurality ofcurved surfaces for near vision of the lens curve has a center ofcurvature on the optical axis and a radius of curvature, which is set sothat a ray being incident on the corresponding curved surface and beingparallel to the optical axis forms an image at a location near a singlefocal point for near vision on the optical axis, the method comprisingthe steps of: bringing an abrasive cloth of a soft material into tightlycontact with a front curve of the multifocal contact lens by fluidpressure; and ausing a relative movement between the front curve and theabrasive cloth to polish the front curve.

According to still further aspect of the present invention, there isprovided a method for producing a multifocal contact lens having a lenscurve formed by altermately arranging a plurality of curved surfaces forfar vision and a plurality of curved surfaces for near vision in theform of concentric zones, wherein each of the plurality of curvedsurfaces for far vision of the lens curve has a center of curvature onan optical axis and a radius of curvature, which is set so that apredetermined principal ray being incident on the corresponding curvedsurface and being parallel to the optical axis forms an image at alocation near a single focal point for far vision on the optical axis,and each of the plurality of curved surfaces for near vision of the lenscurve has a center of curvature on the optical axis and a radius ofcurvature, which is set so that a predetermined principal ray beingincident on the corresponding curved surface and being parallel to theoptical axis forms an image at a location near a single focal point fornear vision on the optical axis, the method comprising the steps of:bringing an abrasive cloth of a soft material into tightly contact witha front curve of the multifocal contact lens by fluid pressure; andcausing a relative movement between the front curve and the abrasivecloth to polish the front curve.

With the aforementioned constructions, the operation of the presentinvention will be described below.

According to the present invention, each of the curved surfaces for farvision of the lens curve may have a center of curvature on the opticalaxis, and a radius of curvature which is set so that a ray beingincident on the corresponding curved surface for far vision and beingparallel to the optical axis forms an image at a location near a singlefocal point for far vision. According to the present invention, there isno severe restriction that all of the radii of curvature of therespective curved surfaces for far vision must be equal to each other,so that it is possible to ensure a high degree of freedom when a lens isdesigned using the ray tracing method.

In addition, according to the present invention, it is required toarrange the centers of curvature of the curved surfaces for far visionon the optical axis. However, according to the present invention, theradii of curvature of the respective curved surfaces for far vision canbe suitably set, and the centers of curvature of the curved surfaces forfar vision can be sequentially moved. Therefore, when the ray tracingmethod is applied, the restriction that the centers of curvature must bearranged on the optical axis is not essential.

With respect to each of the curved surfaces for near vision, theforegoing is the same as that of the curved surfaces for far vision.

Therefore, in comparison with the contact lens shown in FIG. 5 and soforth, it is possible to considerably decrease the restrictions when theray tracing method is applied, and it is possible to design a lens whileensuring a high degree of freedom. In addition, the centers of curvatureof the respective curved surfaces can be arranged on the optical axis,and the spherical aberration can be removed by suitably deriving theshapes of the respective curved surface of the lens curve using the raytracing method.

Alternatively, according to the present invention, each of the curvedsurfaces for far vision of the lens curve may have a center of curvatureon the optical axis, and a radius of curvature which is set so that aray being incident on the corresponding curved surface for far visionand being parallel to the optical axis passes through a location near asingle focal point for far vision. This is the same with respect to eachof the curved surfaces for near vision.

According to the present invention, two methods for selecting apredetermined principal ray are provided.

In one of the two selecting methods, there is selected a ray passingthrough a predetermined location in a zone width of each of all thecurved surfaces for far vision and near vision, which include a curvedsurface for far vision containing the optical axis and a curved surfacefor near vision containing the optical axis, respectively. That is, inthe case of the contact lens shown in FIG. 5 and so forth, the raycorresponding to the optical axis is selected as a principal ray withrespect to the curved surface F1 for far vision and the curved surfaceN1 for near vision. However, according to the present invention,parallel rays passing through a predetermined location in a zone widthof each of the curved surface F1 for far vision and the curved surfaceN1 for near vision are selected as principal rays similar to othercurved surfaces F2, F3, . . . for far vision and curved surfaces N2, N3,. . . which are apart from the optical axis. Thus, also in the case ofthe curved surface F1 for far vision and the curved surface N1 for nearvision, the spherical aberration can be removed similar to the othercurved surfaces F2, F3, . . . for far vision and the other curvedsurfaces N2, N3, . . . for near vision.

Even if the spherical aberrations remain in the curved surface F1 forfar vision and the curved surface N1 for near vision, the distributionof the remaining spherical aberrations can be the same as thedistribution of the spherical aberrations remaining in the other curvedsurfaces F2, F3, . . . for far vision and the other curved surfaces N2,N3, . . . for near vision. As a result, each of the curved surfaces canhave a uniform distribution of spherical aberrations around a focalpoint for far vision or a focal point for near vision, such as aGaussian distribution of spherical aberrations. On the other hand, inthe case of the contact lens shown in FIG. 5 and so forth, thedistribution of spherical aberrations remaining in the curved surface F1for far vision and the curved surface N1 for near vision and thedistribution of spherical aberrations remaining in the other curvedsurface F2, F3, . . . for far vision and the other curved surfaces N2,N3, . . . for near vision are easy to, e.g., a Gaussian distribution anda non-gaussian distribution, respectively.

In the other selecting method, a ray corresponding to the optical axisis selected as a predetermined principal ray with respect to the curvedsurface for far vision containing the optical axis and the curvedsurface for near vision containing the optical axis, and a ray passingthrough a predetermined location in a zone width of each of the curvedsurfaces is selected as a predetermined principal ray with respect tothe other curved surfaces for far vision and near vision. That is,similar to the contact lens shown in FIG. 5 and so forth, a raycorresponding to the optical axis is selected as a principal ray withrespect to the curved surface F1 for far vision and the curved surfaceN1 for near vision. Therefore, the complicated ray tracing can beomitted with respect to the curved surface F1 for far vision and thecurved surface N1 for near vision, so that it is possible to simplifythe design. In addition, even if the ray corresponding to the opticalaxis is selected as the principal ray with respect to the curved surfaceF1 for far vision and the curved surface N1 for near vision, thespherical aberration can be disregarded as long as the zone widths ofthe curved surfaces F for far vision and so forth are small so that theapproximation to paraxial rays can be established.

As mentioned above, according to the present invention, each of theplurality of curved surfaces for far vision of the lens curve may have acenter of curvature on an optical axis and a radius of curvature, whichis set so that a ray being incident on the corresponding curved surfaceand being parallel to the optical axis forms an image at a location neara single focal point for far vision on the optical axis, and each of theplurality of curved surfaces for near vision of the lens curve may havea center of curvature on the optical axis and a radius of curvature,which is set so that a ray being incident on the corresponding curvedsurface and being parallel to the optical axis forms an image at alocation near a single focal point for near vision on the optical axis.Therefore, the spherical aberration of the portions for far vision andnear vision can be removed, so that the user can obtain a clear visualacuity in both of the portions for far vision and near vision.

In addition, the zone width of each of the curved surfaces for farvision may vary in accordance with the distance between thecorresponding curved surface for far vision and the optical axis, andthe zone width of each of the curved surfaces for near vision may varyin accordance with the distance between the corresponding curved surfacefor near vision and the optical axis. Therefore, when the user reads abook or does desk work in a room or the like using the portions for nearvision, the user is easy to see close range if wearing a contact lenshaving a great energy ratio of portion for near vision. In addition, theuser can see far range through the portions for far vision.

When the user takes up a sport or has a drive mainly using the portionsfor far vision outdoors or the like, the user is easy to see far rangeif wearing a contact lens having a great energy ratio of portion for farvision. In addition, the user can see near range through the portionsfor near vision.

According to the present invention, the front curve may be polished bybringing an abrasive cloth of a soft material into tightly contact withthe front curve of the contact lens by fluid pressure to cause therelative movement between the front curve and the abrasive cloth.Therefore, the abrasive cloth can be flexibly fitted to the shape of thecurved surface of the front curve by fluid pressure, so that it ispossible to uniformly polish the contact lens containing the boundarybetween the adjacent curved surfaces for far vision and near vision.

Furthermore, according to the present invention, the lens curve formedby altermately arranging a plurality of curved surfaces for far visionand a plurality of curved surfaces for near vision in the form of zonesconcentric with respect to the optical axis, may be a front curve or abase curve.

In addition, according to the present invention, a curved surface F1 forfar vision may be provided as a curved surface containing the opticalaxis. Alternatively, a curved surface N1 for near vision may besubstituted for the curved surface F1 for far vision as the curvedsurface containing the optical axis.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given herebelow and from the accompanying drawings of thepreferred embodiments of the invention. However, the drawings are notintended to imply limitation of the invention to a specific embodiment,but are for explanation and understanding only.

In the drawings:

FIG. 1 is a sectional view showing centers of curvature of curvedsurfaces for far vision and near vision in the preferred embodiment of acontact lens according to the present invention;

FIG. 2 is an enlarged sectional view showing a part of FIG. 1;

FIG. 3 is a sectional view showing focal points for far vision and nearvision in the preferred embodiment of a contact lens according to thepresent invention;

FIG. 4 is a sectional view showing centers of curvature of curvedsurfaces for far vision and near vision of a conventional contact lens;

FIG. 5 is an enlarged sectional view showing a part of FIG. 4;

FIG. 6 is a sectional view showing focal points for far vision and nearvision of a conventional contact lens;

FIG. 7 is a sectional view showing centers of curvature of curvedsurfaces for far vision and near vision of a conventional contact lens;

FIG. 8 is an enlarged sectional view showing a part of FIG. 7;

FIG. 9 is a plan view of the first through eighth preferred embodimentsof a contact lens according to the present invention;

FIG. 10 is a plan view of the ninth preferred embodiment of a contactlens according to the present invention;

FIG. 11 is a plan view of the tenth preferred embodiment of a contactlens according to the present invention;

FIG. 12 is a plan view of the eleventh preferred embodiment of a contactlens according to the present invention;

FIG. 13 is a plan view of the twelfth preferred embodiment of a contactlens according to the present invention;

FIG. 14 is a plan view of the thirteenth preferred embodiment of acontact lens according to the present invention;

FIG. 15 is a plan view of the fourteenth preferred embodiment of acontact lens according to the present invention;

FIG. 16 is a plan view of the fifteenth preferred embodiment of acontact lens according to the present invention;

FIG. 17 is a plan view of the sixteen preferred embodiment of a contactlens according to the present invention;

FIG. 18 is a plane view of the seventeenth preferred embodiment of acontact lens according to the present invention;

FIG. 19 is a sectional view showing the preferred embodiment of a methodfor producing a contact lens according to the present invention;

FIG. 20 is an enlarged sectional view of a part of FIG. 19;

FIG. 21 is a sectional view of a contact lens mold according to thepresent invention;

FIG. 22 is a view explaining a method for producing a contact lens usinga contact lens mold according to the present invention;

FIG. 23 is a view explaining a method for producing a contact lens usinga contact lens mold according to the present invention; and

FIG. 24 is a view explaining a method for producing a contact lens usinga contact lens mold according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the accompanying drawings, the preferred embodiments ofa multifocal contact lens according to the present invention will bedescribed below.

As shown in FIG. 2, a multifocal contact lens 1 has a front curve 2serving as a lens curve formed by alternately arranging a plurality ofcurved surfaces F1, F2, . . . for far vision and a plurality of curvedsurfaces N1, N2, . . . for near vision in the form of concentric zones,and a base curve 3. It is assumed herein that the optical axis of thecontact lens 1 corresponds to a Z-axis and the positive direction of theZ-axis is from the front curve 2 to the base curve 3. It is also assumedthat an X-axis passes through the vertex P of the contact lens 1 and isperpendicular to the Z-axis.

The shape of the base curve 3 is individually set so as to correspond tothe curved surface of the user's cornea. Furthermore, the base curve 3may serve as a lens curve in place of the front curve 2.

In addition, the position of a focal point F_(F) for far vision iscalculated by a power of the portion for far vision which is derived onthe basis of the values of the set shape of the curved surface of thebase curve. In addition, the position of a focal point F_(N) for nearvision is calculated by a power of the portion for near vision which isderived by subtracting an added power from the power of the portion forfar vision.

In addition, with respect to the curved surfaces F1, F2, . . . for farvision and the curved surfaces N1, N2, . . . for near vision on thefront curve 2, the radii R_(F1), R_(F2), . . . of curvature of theportions for far vision, the radii R_(N1), R_(N2), . . . of curvature ofthe portions for near vision, the positions of centers O_(F1), O_(F2), .. . of curvature of the portions for far vision, and the positions ofcenters O_(N1), O_(N2), . . . of curvature of the portions for nearvision are derived as follows.

That is, the curved surfaces F1, F2, . . . of the respective portionsfor far vision of the front curve 2 have the centers O_(F1), O_(F2), . .. of curvature on the optical axis (Z-axis). In addition, the curvedsurfaces F1, F2, . . . of the respective portions for far vision of thefront curve 2 have radii R_(F1), R_(F2), . . . of curvature,respectively, which are set so that the rays, which are incident on therespective curved surfaces F1, F2, . . . for far vision and which areparallel to the optical axis, form an image at a location near a singlefocal point F_(F) on the optical axis. The respective curved surfacesN1, N2, . . . for near vision of the front curve 2 have centers O_(N1),O_(N2), . . . of curvature on the optical axis. In addition, therespective curved surfaces N1, N2, . . . for near vision of the frontcurve 2 have radii R_(N1), R_(N2), . . . of curvature, which are set sothat the rays, which are incident on the respective curved surfaces N1,N2, . . . for near vision and which are parallel to the optical axis,form an image at a location near a single focal point F_(N) for nearvision on the optical axis.

The positions of centers O_(F1), O_(F2), . . . of curvature and theradii R_(F1), R_(F2), . . . of curvature are set using the ray tracingmethod so that predetermined principal rays, which are incident on therespective curved surfaces F1, F2, . . . for far vision and which areparallel to the optical axis, pass through a location near the singlefocal point F_(F) for far vision on the optical axis. The positions ofcenters O_(N1), O_(N2), . . . of curvature and the radii R_(N1), R_(N2),. . . of curvature are set so that predetermined principal rays, whichare incident on the respective curved surfaces N1, N2, . . . for nearvision and which are parallel to the optical axis, pass through alocation near the single focal point F_(N) of for near vision on theoptical axis. Accordingly, to achieve the above, all the surfaces fornear and far vision are spherical.

Furthermore, in the contact lens 1 illustrated in FIGS. 1 through 3, thezone widths of the respective curved surfaces F1, F2, . . . and N1, N2,. . . for far vision and near vision are wide. In addition, in the caseof the respective curved surfaces F1, F2, . . . for far vision, theportions for far vision are formed as plus lenses in relation to thebase curve 3, and in the case of the respective curved surfaces N1, N2,. . . for near vision, the portions for near vision are also formed asplus lenses in relation to the base curve 3.

The preferred embodiments of a multifocal contact lens according to thepresent invention will be described in detail below.

First, a first proposed radius for the radius R_(F1) of curvature of thecurved surface F1 for far vision is set on the basis of a desired powerof the portion for far vision. Then, as shown in FIG. 2, the position onthe optical axis, which is apart from the vertex P by the length of thefirst proposed radius for the radius R_(F1) of curvature of the portionfor far vision, is derived as a first proposed position for the centerO_(F1) of curvature of the curved surface F1 for far vision.

Then, a circle having the first proposed radius for the radius R_(F1) ofcurvature is described about the first proposed position for the centerO_(F1) of curvature to derive an intersection position of this circlewith a straight line l_(F1) defining the zone width of the curvedsurface F1 for far vision. This intersection position is assumed to be afirst proposed point for the point P_(F1). The curved surface extendingfrom the point P to the point P_(F1) is derived as a first proposal forthe curved surface F1 for far vision.

Specifically, the position of the center O_(F1) of curvature is derivedusing the ray tracing method as follows.

First, parallel rays, which are parallel to the optical axis in the zonewidth between the optical axis and the straight line l_(F1), areselected as predetermined principal rays to be incident on the firstproposal for the curved surface F1 for far vision to derive a positionat which the parallel rays intersect with the optical axis afteroutgoing the base curve 3.

Furthermore, as the predetermined principal rays for the curved surfaceF1 for far vision, parallel rays passing through the center in thedirection of X in the zone width between the optical axis and thestraight line l_(F1) or rays corresponding to the optical axis arepreferably selected. In a case where the parallel rays passing throughthe center in the direction of X in the zone width are selected as thepredetermined principal rays, if the same principal rays are selectedfor all the other curved surfaces for far vision and near vision, it ispossible to reduce the spherical aberration in the case of the curvedsurface F1 for far vision and the curved surface N1 for near visionsimilar to the other curved surfaces F2, F3, . . . for far vision andthe other curved surfaces N2, N3, . . . for near vision. In addition,the distribution of the remaining spherical aberrations can be a uniformdistribution of spherical aberrations over all the curved surfaces forfar vision and near vision. On the other hand, in a case where the rayscorresponding to the optical axis are selected as the predeterminedprincipal rays, the complicated ray tracing can be omitted with respectto the curved surface F1 for far vision and the curved surface N1 fornear vision, so that the design can be simplified.

The predetermined principal rays are selected by any one of theaforementioned methods.

Then, the positions, at which the predetermined principal rays thusselected intersect with the optical axis after outgoing the base curve3, are derived. In a case where these positions are shifted from thefocal point F_(F) for far vision in a negative direction of the Z-axis,the first proposal for the radius R_(F1) of curvature is slightlyincreased, and the increased value is defined as a second proposal forthe radius R_(F1) of curvature. Then, a circle having the same radius asthe second proposal for the radius R_(F1) of curvature is describedabout the point P to derive an intersection point of this circle withthe optical axis. This intersection point is defined as a secondproposal for the position of center O_(F1) of curvature.

In addition, a circle having the same radius as the second proposal forthe radius R_(F1) of curvature is described about the position of thesecond proposal for the position of center O_(F1) of curvature to derivean intersection point of this circle with the straight line l_(F1)defining the zone width of the curved surface F1 for far vision. Thisintersection point is defined as a second proposal for the point P_(F1)and the curved surface extending from the point P to the second proposalfor the point P_(F1) is defined as a second proposal for the curvedsurface F1 for far vision.

On the other hand, in a case where the positions, at which thepredetermined principal rays intersect with the optical axis afteroutgoing the base curve 3, are shifted from the focal point F_(F) forfar vision in a positive direction of the Z-axis, the first proposal forthe radius R_(F1) of curvature is slightly decreased, and the decreasedvalue is defined as a second proposal for the radius R_(F1) ofcurvature. Then, a circle having the same radius as the second proposalfor the radius R_(F1) of curvature is described about the point P toderive an intersection point of this circle with the optical axis. Thisintersection point is defined as a second proposal for the position ofcenter O_(F1) of curvature. Similarly, a circle having the same radiusas the second proposal for the radius R_(F1) of curvature is describedabout the second proposal for the position of center O_(F1) of curvatureto derive an intersection point of this circle with the straight linel_(F1) defining the zone width of the curved surface F1 for far vision.This intersection point is defined as a second proposal for the pointP_(F1), and the curved surface extending from the point P to the secondproposal for the point P_(F1) is defined as a second proposal for thecurved surface F1 for far vision.

As mentioned above, the proposal for the radius R_(F1) of curvature andthe proposal for the center O_(F1) of curvature are sequentiallychanged, and such operations are sequentially repeated until thepredetermined principal rays incoming the front curve 2 and outgoing thebase curve 3 pass through the focal point F_(F) for far vision.

After such ray tracing operations are repeated, when the intersectionpoints of the predetermined principal rays outgoing the base curve 3with the optical axis are incident with the focal point F_(F) for farvision or substantially incident therewith within an allowable range,the proposal for the center O_(F1) of curvature is decided as a trueposition of center O_(F1) of curvature on the optical axis, and theproposal for the radius R_(F1) of curvature is decided as a true radiusR_(F1) of curvature. In addition, the proposal for the intersectionpoint R_(F1) is decided as a true intersection point P_(F1), and theproposal for the curved surface F1 for far vision is decided as a truecurved surface F1 for far vision.

Then, methods for deriving the position of center O_(N1) of curvature,the radius R_(N1) of curvature and the curved surface N1 for near visionon the basis of the aforementioned decided results will be describedbelow.

First, a first proposal for the radius R_(N1) of curvature is set on thebasis of a desired power of the portion for near vision. Then, a circlehaving the same radius as the first proposal for the radius R_(N1) ofcurvature is described about the point P_(F1) on the decided curvedsurface F1 for far vision, to derive an intersection point of thiscircle with the optical axis. This intersection point is defined as afirst proposal for the center O_(N1) of curvature of the curved surfaceN1 for near vision.

Then, a circle having the same radius as the first proposal for theradius R_(N1) of curvature is described about the first proposal for thecenter O_(N1) of curvature to derive an intersection point of thiscircle with the straight line l_(N1) defining the zone width of thecurved surface N1 for near vision. This intersection point is defined asa first proposal for the point P_(N1), and the curved surface extendingfrom the decided point P_(F1) to the first proposal for the point P_(N1)is described as a first proposal for the curved surface N1 for nearvision.

Then, in the same method as that for deriving the position of centerO_(F1) of curvature and the radius R_(F1) of curvature, the position ofcenter O_(F1) of curvature and the radius R_(N1) of curvature arederived using the ray tracing method as follows.

First, rays, which pass through the center of the zone between thestraight line l_(F1) and the straight line l_(N1) in a direction of Xand which are parallel to the optical axis, are selected aspredetermined principal rays. The positions, at which the predeterminedprincipal rays intersect with the optical axis after incoming the firstproposal for the curved surface N1 for near vision and outgoing the basecurve 3, are derived.

In a case where the derived position intersecting with the optical axisis shifted from the focal point F_(N) for near vision in a direction ofZ, the first proposal for the radius R_(N1) of curvature is slightlyincreased as shown in FIG. 2, and the increased value is defined as asecond proposal for the radius R_(N1) of curvature. Then, a circlehaving the same radius as the second proposal for the radius R_(N1) isdescribed about the decided point P_(F1) to derive an intersection pointof this circle with the optical axis. This intersection is defined as aproposal for the center O_(N1) of curvature. In addition, a circlehaving the same radius as the second proposal for the radius R_(N1) ofcurvature is described about the second proposal for the position ofcenter O_(N1) of curvature to derive an intersection point of thiscircle with the straight line l_(N1) defining the zone width of thecurved surface N1 for near vision. This intersection point is defined asa second proposal for the point P_(N1), and the curved surface extendingthe decided point P_(F1) to the point P_(N1) is defined as a secondproposal for the curved surface N1 for near vision.

On the other hand, in a case where the positions, at which thepredetermined principal rays intersect with the optical axis afteroutgoing the base curve 3, are shifted from the focal point F_(N) fornear vision in a positive direction of the Z-axis, the first proposalfor the radius R_(N1) of curvature is slightly decreased, and thedecreased value is defined as a second proposal for the radius R_(N1) ofcurvature. Then, a circle having the same radius as the second proposalfor the radius R_(N1) of curvature is described about the decided pointP_(F1) to derive an intersection point of this circle with the opticalaxis. This intersection point is defined as a second proposal for thecenter O_(N1) of curvature. In addition, a circle having the same radiusas the second proposal for the radius R_(N1) of curvature is describedabout the second proposal for the center O_(N1) of curvature to derivean intersection point of this circle with the straight line l_(N1)defining the zone width of the curved surface N1 for near vision. Thisintersection point is defined as a second proposal for the point P_(N1),and the curved surface extending from the decided point P_(F1) to thesecond proposal for the point P_(N1) is defined as a second proposal forthe curved surface N1 for near vision.

As mentioned above, the proposal for the radius R_(N1) of curvature andthe proposal for the center O_(N1) of curvature are sequentiallychanged. Such operations are sequentially repeated until thepredetermined principal rays pass through the focal point F_(N) for nearvision after incoming the front curve 2 and outgoing the base curve 3.

After such ray tracing operations are repeated, when the intersectionpoint of the predetermined principal rays outgoing the base curve 3 withthe optical axis is incident with the focal point F_(N) for near visionor substantially incident therewith within an allowable range, theproposal for the center O_(N1) of curvature is decided as a trueposition of center O_(N1) of curvature on the optical axis, and theproposal for the radius R_(N1) of curvature is decided as a true radiusR_(N1) of curvature. In addition, the proposal for the intersectionpoint R_(N1) is decided as a true intersection point P_(N1), and theproposal for the curved surface N1 for near vision is decided as a truecurved surface N1 for near vision.

Then, methods for deriving the position of center O_(F2) of curvature,the radius R_(F2) of curvature and the curved surface F2 for far visionon the basis of the aforementioned decided results will be describedbelow.

First, a first proposal for the radius R_(F2) of curvature is set on thebasis of a desired power for far vision. Then, a circle having the sameradius as the first proposal for the radius R_(F2) of curvature isdescribed about the point P_(N1) on the decided curved surface N1 fornear vision to derive an intersection point of this circle with theoptical axis. This intersection point is defined as a first proposal forthe center O_(F2) of curvature of the curved surface F2 for far vision.

Then, a circle having the same radius as the first proposal for theradius R_(F2) of curvature is described about the first proposal for theposition of center O_(F2) of curvature to derive an intersection pointof this circle with the straight line l_(F2) defining the zone width ofthe curved surface F2 for far vision. This intersection point is definedas a first proposal for the point P_(F2), and the curved surfaceextending from the decided point P_(N1) to the first proposal for thepoint P_(F2) is defined as a first proposal for the curved surface F2for far vision.

Then, in the same methods as those for deriving the position of centerO_(F1) of curvature, the radius R_(F1) of curvature and so forth, theposition of center O_(F2) of curvature and the radius R_(F2) ofcurvature are derived using the ray tracing method as follows.

First, rays, which pass through the center of the zone between thestraight line l_(N1) and the straight line l_(F2) in a direction of Xand which are parallel to the optical axis, are selected aspredetermined principal rays. The positions, at which the predeterminedprincipal rays intersect with the optical axis after incoming the firstproposal for the curved surface F2 for far vision and outgoing the basecurve 3, are derived.

In a case where the derived position intersecting with the optical axisis shifted from the focal point F_(F) for far vision in a negativedirection of Z-axis, the first proposal for the radius R_(F2) ofcurvature is slightly increased as shown in FIG. 2, and the increasedvalue is defined as a second proposal for the radius R_(F2) ofcurvature. Then, a circle having the same radius as the second proposalfor the radius R_(F2) is described about the decided point P_(N1) toderive an intersection point of this circle with the optical axis. Thisintersection is defined as a second proposal for the center O_(F2) ofcurvature. In addition, a circle having the same radius as the secondproposal for the radius R_(F2) of curvature is described about thesecond proposal for the center O_(F2) of curvature to derive anintersection point of this circle with the straight line l_(F2) definingthe zone width of the curved surface F2 for far vision. Thisintersection point is defined as a second proposal for the point P_(F2),and the curved surface extending the decided point P_(N1) to the secondproposal for the point P_(F2) is defined as a second proposal for thecurved surface F2 for far vision.

On the other hand, in a case where the positions, at which thepredetermined principal rays intersect with the optical axis afteroutgoing the base curve 3, are shifted from the focal point F_(F) forfar vision in a positive direction of the Z-axis, the first proposal forthe radius R_(F2) of curvature is slightly decreased, and the decreasedvalue is defined as a second proposal for the radius R_(F2) ofcurvature. Then, a circle having the same radius as the second proposalfor the radius R_(F2) of curvature is described about the decided pointP_(N1) to derive an intersection point of this circle with the opticalaxis. This intersection point is defined as a second proposal for thecenter O_(F2) of curvature. In addition, a circle having the same radiusas the second proposal for the radius R_(F2) of curvature is describedabout the second proposal for the position of center O_(F2) of curvatureto derive an intersection point of this circle with the straight linel_(F2) defining the zone width of the curved surface F2 for far vision.This intersection point is defined as a second proposal for the pointP_(F2), and the curved surface extending from the decided point P_(N1)to the second proposal for the point P_(F2) is defined as a secondproposal for the curved surface F2 for far vision.

As mentioned above, the proposal for the radius R_(F2) of curvature andthe proposal for the center O_(F2) of curvature are sequentiallychanged. Such operations are sequentially repeated until thepredetermined principal rays pass through the focal point F_(F) for farvision after incoming the front curve 2 and outgoing the base curve 3.

After such ray tracing operations are repeated, when the intersectionpoint of the predetermined principal rays outgoing the base curve 3 withthe optical axis is incident with the focal point F_(F) for far visionor substantially incident therewith within an allowable range, theproposal for the center O_(F2) of curvature is decided as a trueposition of center O_(F2) of curvature on the optical axis, and theproposal for the radius R_(F2) of curvature is decided as a true radiusR_(F2) of curvature. In addition, the proposal for the intersectionpoint P_(F2) with the straight line l_(F2) defining the zone width isdecided as a true intersection point, and the proposal for the curvedsurface F2 for far vision is decided as a true curved surface F2 for farvision.

By the same operations, the other centers O_(F3), O_(F4), . . . andO_(N2), O_(N3), . . . , the other radii R_(F3), R_(F4), . . . andR_(N2), R_(N3), . . . of curvature, and the other curved surfaces F3,F4, . . . and N2, N3, . . . are derived.

The characteristics of the contact lens 1 thus derived are shown inFIGS. 1 and 3. As shown in FIG. 1, all of the centers O_(F1), O_(F2), .. . of curvature of the portions for far vision and the centers O_(N1),O_(N2), . . . are located on the optional axis.

In addition, as schematically shown in FIG. 3, the rays, which areparallel to the rays incoming the respective curved surface F1, F2, . .. of curvature for far vision of the front curve 2, form an image at alocation near the single focal point F_(F) for far vision on the opticalaxis, and the rays, which are parallel to the rays incoming therespective curved surfaces N1, N2, . . . of curvature of for near visionof the front curve 2, form an image at a location near the single focalpoint F_(F) for near vision on the optical axis.

As mentioned above, according to the present invention, there is removedthe severe restrictions that all the radii of curvature of therespective curved surfaces F1, F2, . . . for far vision must be equal toeach other, and all of the centers O_(F1), O_(F2), . . . of curvature ofthe respective curved surfaces F1, F2, . . . for far vision are locatedon the optical axis. Therefore, it is possible to ensure a high degreeof freedom when a lens is designed by the ray tracing method, and it ispossible to remove spherical aberrations of the respective portions forfar vision.

Similarly, there is removed the severe restrictions that all the radiiof curvature of the respective curved surfaces N1, N2, . . . for nearvision must be equal to each other, and all of the respective centersO_(N1), O_(N2), . . . of curvature are located on the optical axis.Therefore, it is possible to ensure a high degree of freedom when a lensis designed by the ray tracing method, and it is possible to removespherical aberrations of the respective portions for near vision.

As a result, the user can obtain a clear visual acuity in both ofportions for far vision and near vision.

In addition, in the cases of the curved surface F1 for far vision andthe curved surface N1 for near vision as in the case of the other curvedsurfaces F2, F3, . . . for far vision and the other curved surfaces N2,N3, . . . for near vision, it is possible to remove sphericalaberrations by selecting parallel rays passing through predeterminedpositions, e.g., the centers, in the respective zone width as thepredetermined principal rays with respect to all of the curved surfacesF1, F2, . . . for far vision and the curved surfaces N1, N2, . . . fornear vision. In addition, even if spherical aberrations remain in thecurved surface F1 for far vision and the curved surface N1 for nearvision, the distribution of the remaining spherical aberrations can bethe same as the distribution of spherical aberrations remaining in theother curved surfaces F2, F3, . . . for far vision and the other curvedsurfaces N2, N3, . . . for near vision, and the respective curvedsurfaces can have a uniform distribution of spherical aberrations aroundthe focal point for far vision or the focal point for near vision so asto have, e.g., a Gaussian distribution of spherical aberrations.

In addition, if rays corresponding to the optical axis are selected asthe predetermined principal rays with respect to the curved surface F1for far vision containing the optical axis or the curved surface N1 fornear vision containing the optical axis, or if rays passing throughpredetermined positions, e.g., centers, in the zone widths of therespective curved surfaces are selected as the predetermined principalrays with respect to the other curved surfaces F2, F3, . . . for farvision and the other curved surfaces N2, N3, . . . for near vision, itis possible to omit the complicated ray tracing, so that the design canbe simplified.

In addition, as can be seen from FIG. 1, as the distances between therespective curved surfaces F1, F2, . . . for far vision of the frontcurve 2 and the optical axis are increased, the radii of curvature ofthe curved surfaces F1, F2, . . . for far vision are increased, and thedistances between the centers O_(F1), O_(F2), . . . of curvature thereofand the front curve 2 are increased. In addition, as the distancesbetween the respective curved surfaces N1, N2, . . . for near vision ofthe front curve 2 and the optical axis are increased, the radii ofcurvature of the curved surfaces N1, N2, . . . for near vision areincreased, and the distances between the centers O_(N1), O_(N2), . . .of curvature thereof and the front curve 2 are increased.

Furthermore, as mentioned above, in the contact lens 1 illustrated inFIGS. 1 through 3, the zone widths of the respective curved surfaces F1,F2, . . . for far vision and N1, N2, . . . for near vision are wide. Inaddition, the portions for far vision are formed as plus lenses inrelation to the base curve 3, and the portions for near vision are alsoformed as plus lenses in relation to the base curve 3.

However, the present invention should not be limited thereto, but theundermentioned combinations of plus lenses and minus lenses for theportions for far vision and near vision can remove spherical aberration.

In addition, in the undermentioned combinations of plus lenses and minuslenses for the portions for far vision and near vision, there are thefollowing characteristics.

In a case where the zone widths of the respective curved surfaces forfar vision and near vision are wide, when the portions for far visionare minus lenses and the portions for near vision are plus lenses, orwhen both of the portions for far vision and near vision are minuslenses, the spherical aberrations of the respective curved surfaces forfar vision and near vision can be removed.

However, when the portions for far vision are minus lenses and theportions for near vision are plus lenses, as the distances between therespective curved surfaces F1, F2, . . . for far vision of the frontcurve 2 and the optical axis are increased, the radii of curvature aredecreased, and the distances between the centers O_(F1), O_(F2), . . .of curvature and the front curve 2 are decreased. In addition, as thedistances between the respective curved surfaces N1, N2, . . . for nearvision of the front curve 2 and the optical axis are increased, theradii of curvature are increased, and the distances between the centersO_(N1), O_(N2), . . . of curvature and the front curve 2 are increased.

In addition, when both of the portions for far vision and near visionare minus lenses, as the distances between the respective curvedsurfaces F1, F2, . . . for far vision of the front curve 2 and theoptical axis are increased, the radii of curvature are decreased, andthe distances between the centers O_(F1), O_(F2), . . . of curvature andthe front curve 2 are decreased. In addition, as the distances betweenthe respective curved surfaces N1, N2, . . . for near vision of thefront curve 2 and the optical axis are increased, the radii of curvatureare decreased, and the distances between the centers O_(N1), O_(N2), . .. of curvature and the front curve 2 are decreased while the centers arelocated on the optical axis.

On the other hand, in a case where the zone widths of the respectivecurved surfaces for far vision and near vision are narrow, when both ofthe portions for far vision and near vision are plus lenses, even if thedistances between the respective curved surfaces F1, F2, . . . for farvision of the front curve 2 and the optical axis are increased, theradii of curvature are substantially the same, but the distances betweenthe centers O_(F1), O_(F2), . . . of curvature and the front curve 2 areincreased. In addition, as the distances between the respective curvedsurfaces N1, N2, . . . for near vision of the front curve 2 and theoptical axis are increased, the radii of curvature are increased, andthe distances between the centers O_(N1), O_(N2), . . . of curvature andthe front curve 2 are increased while the centers are located on theoptical axis.

In addition, when the portions for far vision are minus lenses and theportions for near vision are plus lenses, as the distances between therespective curved surfaces F1, F2, . . . for far vision of the frontcurve 2 and the optical axis are increased, the radii of curvature aredecreased, and the centers O_(F1), O_(F2), . . . of curvature arealtermately located on the optical axis at a far position and a nearposition from the front curve 2. In addition, as the distances betweenthe respective curved surfaces N1, N2, . . . for near vision of thefront curve 2 and the optical axis are increased, the radii of curvatureare substantially the same, but the distances between the centersO_(N1), O_(N2), . . . of curvature and the front curve 2 are increased.

Using numeric values, the improvements of spherical aberration accordingto the present invention compared with the conventional design will bedescribed below.

The contact lens used herein is a water-containing soft contact leans,and the radius of curvature of the base curve 3 is 8.0 mm in a moisturestate. As shown in FIG. 9, the zone widths of the portions for farvision are equal to each other to be 0.5 mm, and the zone widths of theportions for near vision are equal to each other to be 1.0 mm. In thiscase, the area ratio in the optical zone is 60% with respect to theportions for far vision and 40% with respect to the portions for nearvision.

In the tables showing the comparative results, the terms "first zone","second zone", "third zone", "fourth zone", . . . are used in the orderof the first zone for far vision, the first zone for near vision, thesecond zone for far vision, the second zone for near vision, . . .outside from the center of the contact lens, respectively. Therefore, inthe respective tables, the term "first zone" means the first zone forfar vision comprising the curved surface F1 for far vision and the basecurve 3, the term "second zone" means the first zone for near visioncomprising the curved surface N1 for near vision and the base curve 3,the term "third zone" means the second zone for far vision comprisingthe curved surface F2 for far vision and the base curve 3, and the term"fourth zone" means the second zone for near vision comprising thecurved surface N2 for near vision and the base curve 3. In addition, theradii of curvature, the X-coordinate and Y-coordinate of the curvedsurfaces corresponding to the first, second, third, fourth, zones,indicate the radii of curvature of the respective curved surfaces F1,N1, F2, N2, . . . , and the X-coordinate and Y-coordinate of the centersO_(F1), O_(N2), O_(F2), O_(N2), . . . of curvature of the respectivecurved surfaces F1, N1, F2, N2, . . . , respectively.

In addition, the amount improved by the design of the present inventioncompared with the conventional design is shown by the improved amount ofspherical aberration. The improved amount of spherical aberration isshown by the difference in spherical aberration in the fifth zone (F3)for example.

First, referring to FIG. 9, the first through eighth preferredembodiments are shown in Tables 1 through 8. Furthermore, in thefollowing tables, the radius of curvature is expressed by R, and theX-coordinate and Y-coordinate are expressed by X and Y, respectively.

(1) Example 1

When the power for far vision is +3.00D (diopter), the added power fornear vision is +2.00D and the zone widths of the portions for far visionand near vision are 1 mm, the comparative results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                               The Present Invention                                                                       Conventional Design                                             R(mm) X(mm)   Z(mm)   R(mm) X(mm) Z(mm)                                ______________________________________                                        First Zone                                                                             7.70    0.00000 7.69745                                                                             7.70  0.00000                                                                             7.69745                            Second Zone                                                                            7.48    0.00000 7.69745                                                                             7.46  0.00300                                                                             7.46961                            Third Zone                                                                             7.72    0.00000 7.74434                                                                             7.70  0.00750                                                                             7.72237                            Fourth Zone                                                                            7.58    0.00000 7.60454                                                                             7.46  0.05625                                                                             7.49658                            Fifth Zone                                                                             7.82    0.00000 7.88856                                                                             7.70  0.07050                                                                             7.78824                            Improved Amount of                                                                              0.36D                                                       Spherical Aberration                                                          ______________________________________                                    

As can be seen from Table 1, in the portions for far vision, the radiusof curvature is monotonously increased and the Z-coordinate is alsomonotonously increased. On the other hand, in the portions for nearvision, the radius of curvature is monotonously increased and theZ-coordinate is also monotonously increased. In addition, in theportions for both of far vision and near vision, all of theX-coordinates are zero. According to the present invention, thespherical aberration was improved by 0.36D

(2) Example 2

When the power for far vision is -3.00D (diopter), the added power fornear vision is +4.00D and the zone widths of the portions for far visionand near vision are 1 mm, the comparative results are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        The Present Invention                                                                             Conventional Design                                       R(mm)       X(mm)   Z(mm)   R(mm) X(mm)  Z(mm)                                ______________________________________                                        First Zone                                                                            8.43    0.00000 8.42885                                                                             8.43    0.00000                                                                            8.42885                            Second Zone                                                                           7.89    0.00000 7.89743                                                                             7.89    0.00150                                                                            7.89013                            Third Zone                                                                            8.39    0.00000 8.41754                                                                             8.43  -0.01188                                                                             8.45522                            Fourth Zone                                                                           7.93    0.00000 7.92664                                                                             7.89    0.01725                                                                            7.89155                            Fifth Zone                                                                            8.30    0.00000 8.36429                                                                             8.43  -0.07096                                                                             8.47421                            Improved Amount of                                                                             0.69D                                                        Spherical Aberration                                                          ______________________________________                                    

As can be seen from Table 2, in the portions for far vision, the radiusof curvature is monotonously decreased and the Z-coordinate ismonotonously increased. On the other hand, in the portions for nearvision, the radius of curvature is monotonously increased and theZ-coordinate is also monotonously increased. In addition, in theportions for both of far vision and near vision, all of theX-coordinates are zero. According to the present invention, thespherical aberration was improved by 0.69D.

(3) Example 3

When the power for far vision is -3.00D (diopter), the added power fornear vision is +2.00D and the zone widths of the portions for far visionand near vision are 1 mm, the comparative results are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        The Present Invention                                                                             Conventional Design                                       R(mm)       X(mm)   Z(mm)   R(mm) X(mm)  Z(mm)                                ______________________________________                                        First Zone                                                                            8.43    0.00000 8.43147                                                                             8.43    0.00000                                                                            8.42768                            Second Zone                                                                           8.15    0.00000 8.15344                                                                             8.15    0.00150                                                                            8.15344                            Third Zone                                                                            8.39    0.00000 8.41006                                                                             8.43  -0.01188                                                                             8.44775                            Fourth Zone                                                                           8.14    0.00000 8.14723                                                                             8.15  -0.00487                                                                             8.15778                            Fifth Zone                                                                            8.30    0.00000 8.34659                                                                             8.43  -0.06771                                                                             8.45207                            Improved Amount of                                                                             0.31D                                                        Spherical Aberration                                                          ______________________________________                                    

As can be seen from Table 3, in the portions for far vision, the radiusof curvature is monotonously decreased and the Z-coordinate is alsomonotonously decreased. On the other hand, in the portions for nearvision, the radius of curvature is monotonously decreased and theZ-coordinate is also monotonously decreased. In addition, in theportions for both of far vision and near vision, all of theX-coordinates are zero. According to the present invention, thespherical aberration was improved by 0.31D.

(4) Example 4

When the power for far vision is +5.00D (diopter), the added power fornear vision is +4.00D and the zone widths of the portions for far visionand near vision are 1 mm, the comparative results are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                               The Present Invention                                                                       Conventional Design                                             R(mm) X(mm)   Z(mm)   R(mm) X(mm) Z(mm)                                ______________________________________                                        First Zone                                                                             7.47    0.00000 7.47213                                                                             7.47  0.00000                                                                             7.47213                            Second Zone                                                                            7.07    0.00000 7.07589                                                                             7.04  0.00638                                                                             7.04723                            Third Zone                                                                             7.52    0.00000 7.55200                                                                             7.47  0.01650                                                                             7.50563                            Fourth Zone                                                                            7.25    0.00000 7.26278                                                                             7.04  0.10013                                                                             7.08099                            Fifth Zone                                                                             7.67    0.00000 7.76545                                                                             7.47  0.11400                                                                             7.60798                            Improved Amount of                                                                              0.64D                                                       Spherical Aberration                                                          ______________________________________                                    

As can be seen from Table 4, in the portions for far vision, the radiusof curvature is monotonously increased and the Z-coordinate is alsomonotonously increased. On the other hand, in the portions for nearvision, the radius of curvature is monotonously increased and theZ-coordinate is also monotonously increased. In addition, in theportions for both of far vision and near vision, all of theX-coordinates are zero. According to the present invention, thespherical aberration was improved by 0.64D.

(5) Example 5

When the power for far vision is -5.00D (diopter), the added power fornear vision is +2.00D and the zone widths of the portions for far visionand near vision are 1 mm, the comparative results are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        The Present Invention                                                                             Conventional Design                                       R(mm)       X(mm)   Z(mm)   R(mm) X(mm)  Z(mm)                                ______________________________________                                        First Zone                                                                            8.72    0.00000 8.72457                                                                             8.72    0.00000                                                                            8.72457                            Second Zone                                                                           8.14    0.00000 8.14866                                                                             8.14    0.00000                                                                            8.14866                            Third Zone                                                                            8.65    0.00000 8.67507                                                                             8.72  -0.02250                                                                             8.74906                            Fourth Zone                                                                           8.14    0.00000 8.13482                                                                             8.14  -0.00433                                                                             8.14310                            Fifth Zone                                                                            8.48    0.00000 8.54035                                                                             8.72  -0.12837                                                                             8.74566                            Improved Amount of                                                                             0.53D                                                        Spherical Aberration                                                          ______________________________________                                    

As can be seen from Table 5, in the portions for far vision, the radiusof curvature is monotonously decreased and the Z-coordinate is alsomonotonously decreased. On the other hand, in the portions for nearvision, the radii of curvature are substantially the same and theZ-coordinate is monotonously decreased. In addition, in the portions forboth of far vision and near vision, all of the X-coordinates are zero.According to the present invention, the spherical aberration wasimproved by 0.53D.

(6) Example 6

When the power for far vision is +3.00D (diopter), the added power fornear vision is +2.00D and the zone widths of the portions for far visionand near vision are 0.5 mm, the comparative results are shown in Table6.

                  TABLE 6                                                         ______________________________________                                               The Present Invention                                                                       Conventional Design                                             R(mm) X(mm)   Z(mm)   R(mm) X(mm) Z(mm)                                ______________________________________                                        First Zone                                                                             7.70    0.00000 7.69691                                                                             7.70  0.00000                                                                             7.69691                            Second Zone                                                                            7.46    0.00000 7.47028                                                                             7.46  0.00000                                                                             7.47028                            Third Zone                                                                             7.71    0.00000 7.71537                                                                             7.70  0.00000                                                                             7.71537                            Fourth Zone                                                                            7.48    0.00000 7.50289                                                                             7.46  0.00450                                                                             7.48544                            Fifth Zone                                                                             7.71    0.00000 7.74663                                                                             7.70  0.00338                                                                             7.73665                            Improved Amount of                                                                              0.02D                                                       Spherical Aberration                                                          ______________________________________                                    

As can be seen from Table 6, in the portions for far vision, the radiiof curvature are substantially the same and the Z-coordinate ismonotonously increased. On the other hand, in the portions for nearvision, the radius of curvature is monotonously increased and theZ-coordinate is also monotonously decreased. In addition, in theportions for both of far vision and near vision, all of theX-coordinates are zero. According to the present invention, thespherical aberration was improved by 0.02D.

(7) Example 7

When the power for far vision is -3.00D (diopter), the added power fornear vision is +4.00D and the zone widths of the portions for far visionand near vision are 0.5 mm, the comparative results are shown in Table7.

                  TABLE 7                                                         ______________________________________                                        The Present Invention                                                                             Conventional Design                                       R(mm)       X(mm)   Z(mm)   R(mm) X(mm)  Z(mm)                                ______________________________________                                        First Zone                                                                            8.43    0.00000 8.43147                                                                             8.43    0.00000                                                                            8.43147                            Second Zone                                                                           7.89    0.00000 7.89570                                                                             7.89    0.00000                                                                            7.89570                            Third Zone                                                                            8.43    0.00000 8.44577                                                                             8.43  -0.00125                                                                             8.45124                            Fourth Zone                                                                           7.89    0.00000 7.90862                                                                             7.89    0.00038                                                                            7.90722                            Fifth Zone                                                                            8.39    0.00000 8.43660                                                                             8.43  -0.01029                                                                             8.47344                            Improved Amount of                                                                             0.05D                                                        Spherical Aberration                                                          ______________________________________                                    

As can be seen from Table 7, in the portions for far vision, the radiusof curvature is monotonously decreased and the Z-coordinate isrepeatedly decreased after being increased once. On the other hand, inthe portions for near vision, the radii of curvature are substantiallythe same and the Z-coordinate is monotonously increased. In addition, inthe portions for both of far vision and near vision, all of theX-coordinates are zero. According to the present invention, thespherical aberration was improved by 0.05D.

(8) Example 8

When the power for far vision is -3.00D (diopter), the added power fornear vision is +2.00D and the zone widths of the portions for far visionand near vision are 0.5 mm, the comparative results are shown in Table8.

                  TABLE 8                                                         ______________________________________                                        The Present Invention                                                                             Conventional Design                                       R(mm)       X(mm)   Z(mm)   R(mm) X(mm)  Z(mm)                                ______________________________________                                        First Zone                                                                            8.43    0.00000 8.43031                                                                             8.43    0.00000                                                                            8.43031                            Second Zone                                                                           8.15    0.00000 8.15764                                                                             8.15    0.00000                                                                            8.15764                            Third Zone                                                                            8.42    0.00000 8.44303                                                                             8.43  -0.00125                                                                             8.44850                            Fourth Zone                                                                           8.15    0.00000 8.16983                                                                             8.15  -0.00054                                                                             8.17163                            Fifth Zone                                                                            8.39    0.00000 8.42980                                                                             8.43  -0.00975                                                                             8.46680                            Improved Amount of                                                                             0.05D                                                        Spherical Aberration                                                          ______________________________________                                    

As can be seen from Table 8, in the portions for far vision, the radiusof curvature is monotonously decreased and the Z-coordinate isrepeatedly decreased after being increased once. On the other hand, inthe portions for near vision, the radii of curvature are substantiallythe same and the Z-coordinate is monotonously increased. In addition, inthe portions for both of far vision and near vision, all of theX-coordinates are zero. According to the present invention, thespherical aberration was improved by 0.05D.

As shown in Examples 1 through 8, according to the present invention, ina case where the zone widths in the portions for both of far vision andnear vision are substantially the same, the spherical aberration can besignificantly improved over the whole optical zones in comparison withthe conventional design. In particular, when the zone widths in theportions for far vision and near vision are wide and/or when theabsolute value of the power for far vision is great, the sphericalaberration can be greatly improved. While only the improved amount ofspherical aberration in the fifth zone has been shown as an example, thespherical aberrations in all of the second zone or more can be improvedaccording to the present invention although the same sphericalaberrations occur in all of the second zone or more in the conventionaldesign.

In the aforementioned preferred embodiments, while the zone widths forboth of far vision and near vision have been substantially the same, thedifferent zone widths for far vision and near vision may be used.

Referring to FIGS. 10 through 18 and using numeric values, theimprovements of spherical aberration according to the present inventioncompared with the conventional design will be described below.

The contact lens used herein is a water-containing soft contact leans,and the radius of curvature of the base curve 3 is 8.0 mm in a moisturestate. In addition, the power for far vision is +3.00D (diopter) and theadded power for near vision is +2.00D. Furthermore, the first zone forfar vision, the first zone for near vision, the second zone for farvision, the second zone for near vision, . . . outside from the centerof the contact lens 1, are defined by the terms "first zone", "secondzone", "third zone", "fourth zone", . . . , respectively. In addition,the respective curved surfaces are defined by F1, N1, F2, N2, . . . andthe respective centers of curvature are defined by O_(F1) O_(N2),O_(F2), O_(N2). . . . Furthermore, the improved amount of sphericalaberration according to the present invention compared with theconventional design is shown by the difference in spherical aberrationin the fifth zone (F3).

Then, referring to FIGS. 10 through 18, the ninth through seventeenthpreferred embodiments are shown in Tables 9 through 17.

(9) Example 9 (see FIG. 10)

First Zone Width for Far Vision: 0.50 mm

First Zone Width for Near Vision: 0.50 mm

Second Zone Width for Far Vision: 0.75 mm

Second Zone Width for Near Vision: 1.00 mm

Third Zone Width for Far Vision: 1.00 mm

Area Ratio in Optical Zone (Portion for Far Vision: Portion for NearVision) 62:38

                  TABLE 9                                                         ______________________________________                                               The Present Invention                                                                       Conventional Design                                             R(mm) X(mm)   Z(mm)   R(mm) X(mm) Z(mm)                                ______________________________________                                        First Zone                                                                             7.70    0.00000 7.69691                                                                             7.70  0.00000                                                                             7.69691                            Second Zone                                                                            7.46    0.00000 7.47028                                                                             7.46  0.00000                                                                             7.47028                            Third Zone                                                                             7.71    0.00000 7.71537                                                                             7.70  0.00000                                                                             7.71537                            Fourth Zone                                                                            7.50    0.00000 7.52384                                                                             7.46  0.01238                                                                             7.48560                            Fifth Zone                                                                             7.75    0.00000 7.79298                                                                             7.70  0.02063                                                                             7.74874                            Improved Amount of                                                                              0.14D                                                       Spherical Aberration                                                          ______________________________________                                    

As can be seen from Table 9, in the portions for far vision, the radiusof curvature is monotonously increased and the Z-coordinate is alsomonotonously increased. On the other hand, in the portions for nearvision, the radius of curvature is monotonously increased and theZ-coordinate is also monotonously increased. In addition, in theportions for both of far vision and near vision, all of theX-coordinates are zero. According to the present invention, thespherical aberration was improved by 0.14D.

(10) Example 10 (see FIG. 11)

First Zone Width for Far Vision: 0.50 mm

First Zone Width for Near Vision: 0.75 mm

Second Zone Width for Far Vision: 0.75 mm

Second Zone Width for Near Vision: 0.75 mm

Third Zone Width for Far Vision: 1.00 mm

Area Ratio in Optical Zone (Portion for Far Vision: Portion for NearVision) 65:35

                  TABLE 10                                                        ______________________________________                                               The Present Invention                                                                       Conventional Design                                             R(mm) X(mm)   Z(mm)   R(mm) X(mm) Z(mm)                                ______________________________________                                        First Zone                                                                             7.70    0.00000 7.69691                                                                             7.70  0.00000                                                                             7.69691                            Second Zone                                                                            7.46    0.00000 7.47028                                                                             7.46  0.00000                                                                             7.47028                            Third Zone                                                                             7.71    0.00000 7.71641                                                                             7.70  0.00000                                                                             7.71641                            Fourth Zone                                                                            7.51    0.00000 7.52905                                                                             7.46  0.01463                                                                             7.48554                            Fifth Zone                                                                             7.75    0.00000 7.79202                                                                             7.70  0.02063                                                                             7.74782                            Improved Amount of                                                                              0.14D                                                       Spherical Aberration                                                          ______________________________________                                    

As can be seen from Table 10, in the portions for far vision, the radiusof curvature is monotonously increased and the Z-coordinate is alsomonotonously increased. On the other hand, in the portions for nearvision, the radius of curvature is monotonously increased and theZ-coordinate is also monotonously increased. In addition, in theportions for both of far vision and near vision, all of theX-coordinates are zero. According to the present invention, thespherical aberration was improved by 0.14D.

(11) Example 11 (see FIG. 12)

First Zone Width for Far Vision: 0.50 mm

First Zone Width for Near Vision: 1.00 mm

Second Zone Width for Far Vision: 0.75 mm

Second Zone Width for Near Vision: 0.50 mm

Third Zone Width for Far Vision: 1.00 mm

Area Ratio in Optical Zone (Portion for Far Vision: Portion for NearVision) 68:32

                  TABLE 11                                                        ______________________________________                                               The Present Invention                                                                       Conventional Design                                             R(mm) X(mm)   Z(mm)   R(mm) X(mm) Z(mm)                                ______________________________________                                        First Zone                                                                             7.70    0.00000 7.69691                                                                             7.70  0.00000                                                                             7.69691                            Second Zone                                                                            7.46    0.00000 7.47028                                                                             7.46  0.00000                                                                             7.47028                            Third Zone                                                                             7.70    0.00000 7.72361                                                                             7.70  0.00000                                                                             7.71829                            Fourth Zone                                                                            7.52    0.00000 7.53576                                                                             7.46  0.01463                                                                             7.48618                            Fifth Zone                                                                             7.75    0.00000 7.79150                                                                             7.70  0.02063                                                                             7.74729                            Improved Amount of                                                                              0.14D                                                       Spherical Aberration                                                          ______________________________________                                    

As can be seen from Table 11, in the portions for far vision, the radiusof curvature is monotonously increased and the Z-coordinate is alsomonotonously increased. On the other hand, in the portions for nearvision, the radius of curvature is monotonously increased and theZ-coordinate is also monotonously increased. In addition, in theportions for both of far vision and near vision, all of theX-coordinates are zero. According to the present invention, thespherical aberration was improved by 0.14D.

(12) Example 12 (see FIG. 13)

First Zone Width for Far Vision: 0.75 mm

First Zone Width for Near Vision: 0.50 mm

Second Zone Width for Far Vision: 0.75 mm

Second Zone Width for Near Vision: 1.00 mm

Third Zone Width for Far Vision: 0.75 mm

Area Ratio in Optical Zone (Portion for Far Vision: Portion for NearVision) 58:42

                  TABLE 12                                                        ______________________________________                                               The Present Invention                                                                       Conventional Design                                             R(mm) X(mm)   Z(mm)   R(mm) X(mm) Z(mm)                                ______________________________________                                        First Zone                                                                             7.70    0.00000 7.69528                                                                             7.70  0.00000                                                                             7.69528                            Second Zone                                                                            7.46    0.00000 7.47021                                                                             7.46  0.00038                                                                             7.46800                            Third Zone                                                                             8.70    0.00000 7.71588                                                                             7.70  0.00075                                                                             7.71412                            Fourth Zone                                                                            7.52    0.00000 7.53677                                                                             7.46  0.01838                                                                             7.48438                            Fifth Zone                                                                             8.75    0.00000 7.79852                                                                             7.70  0.02363                                                                             7.75001                            Improved Amount of                                                                              0.13D                                                       Spherical Aberration                                                          ______________________________________                                    

As can be seen from Table 12, in the portions for far vision, the radiusof curvature is monotonously increased and the Z-coordinate is alsomonotonously increased. On the other hand, in the portions for nearvision, the radius of curvature is monotonously increased and theZ-coordinate is also monotonously increased. In addition, in theportions for both of far vision and near vision, all of theX-coordinates are zero. According to the present invention, thespherical aberration was improved by 0.13D.

(13) Example 13 (see FIG. 14)

First Zone Width for Far Vision: 0.75 mm

First Zone Width for Near Vision: 0.75 mm

Second Zone Width for Far Vision: 0.75 mm

Second Zone Width for Near Vision: 0.75 mm

Third Zone Width for Far Vision: 0.75 mm

Area Ratio in Optical Zone (Portion for Far Vision: Portion for NearVision) 60:40

                  TABLE 13                                                        ______________________________________                                               The Present Invention                                                                       Conventional Design                                             R(mm) X(mm)   Z(mm)   R(mm) X(mm) Z(mm)                                ______________________________________                                        First Zone                                                                             7.70    0.00000 7.69528                                                                             7.70  0.00000                                                                             7.69528                            Second Zone                                                                            7.47    0.00000 7.47028                                                                             7.46  0.00075                                                                             7.46804                            Third Zone                                                                             7.70    0.00000 7.72303                                                                             7.70  0.00188                                                                             7.71590                            Fourth Zone                                                                            7.52    0.00000 7.54363                                                                             7.46  0.02175                                                                             7.48505                            Fifth Zone                                                                             7.75    0.00000 7.80072                                                                             7.70  0.02475                                                                             7.74970                            Improved Amount of                                                                              0.12D                                                       Spherical Aberration                                                          ______________________________________                                    

As can be seen from Table 13, in the portions for far vision, the radiusof curvature is monotonously increased and the Z-coordinate is alsomonotonously increased. On the other hand, in the portions for nearvision, the radius of curvature is monotonously increased and theZ-coordinate is also monotonously increased. In addition, in theportions for both of far vision and near vision, all of theX-coordinates are zero. According to the present invention, thespherical aberration was improved by 0.12D.

(14) Example 14 (see FIG. 15)

First Zone Width for Far Vision: 0.75 mm

First Zone Width for Near Vision: 1.00 mm

Second Zone Width for Far Vision: 0.75 mm

Second Zone Width for Near Vision: 0.50 mm

Third Zone Width for Far Vision: 0.75 mm

Area Ratio in Optical Zone (Portion for Far Vision: Portion for NearVision) 63:37

                  TABLE 14                                                        ______________________________________                                               The Present Invention                                                                       Conventional Design                                             R(mm) X(mm)   Z(mm)   R(mm) X(mm) Z(mm)                                ______________________________________                                        First Zone                                                                             7.70    0.00000 7.69528                                                                             7.70  0.00000                                                                             7.69528                            Second Zone                                                                            7.47    0.00000 7.47733                                                                             7.46  0.00188                                                                             7.46816                            Third Zone                                                                             7.70    0.00000 7.73124                                                                             7.70  0.00375                                                                             7.71783                            Fourth Zone                                                                            7.53    0.00000 7.55057                                                                             7.46  0.02550                                                                             7.48557                            Fifth Zone                                                                             7.75    0.00000 7.79965                                                                             7.70  0.02475                                                                             7.74863                            Improved Amount of                                                                              0.12D                                                       Spherical Aberration                                                          ______________________________________                                    

As can be seen from Table 14, in the portions for far vision, the radiusof curvature is monotonously increased and the Z-coordinate is alsomonotonously increased. On the other hand, in the portions for nearvision, the radius of curvature is monotonously increased and theZ-coordinate is also monotonously increased. In addition, in theportions for both of far vision and near vision, all of theX-coordinates are zero. According to the present invention, thespherical aberration was improved by 0.12D.

(15) Example 15 (see FIG. 16)

First Zone Width for Far Vision: 1.00 mm

First Zone Width for Near Vision: 0.50 mm

Second Zone Width for Far Vision: 0.75 mm

Second Zone Width for Near Vision: 1.00 mm

Third Zone Width for Far Vision: 0.50 mm

Area Ratio in Optical Zone (Portion for Far Vision: Portion for NearVision) 52:48

                  TABLE 15                                                        ______________________________________                                               The Present Invention                                                                       Conventional Design                                             R(mm) X(mm)   Z(mm)   R(mm) X(mm) Z(mm)                                ______________________________________                                        First Zone                                                                             7.70    0.00000 7.69745                                                                             7.70  0.00000                                                                             7.69745                            Second Zone                                                                            7.47    0.00000 7.47482                                                                             7.46  0.00000                                                                             7.46935                            Third Zone                                                                             7.70    0.00000 7.72217                                                                             7.70  0.00150                                                                             7.71722                            Fourth Zone                                                                            7.53    0.00000 7.55246                                                                             7.46  0.02550                                                                             7.48750                            Fifth Zone                                                                             7.76    0.00000 7.80729                                                                             7.70  0.02625                                                                             8.75532                            Improved Amount of                                                                              0.07D                                                       Spherical Aberration                                                          ______________________________________                                    

As can be seen from Table 15, in the portions for far vision, the radiusof curvature is monotonously increased and the Z-coordinate is alsomonotonously increased. On the other hand, in the portions for nearvision, the radius of curvature is monotonously increased and theZ-coordinate is also monotonously increased. In addition, in theportions for both of far vision and near vision, all of theX-coordinates are zero. According to the present invention, thespherical aberration was improved by 0.07D.

(16) Example 16 (see FIG. 17)

First Zone Width for Far Vision: 1.00 mm

First Zone Width for Near Vision: 0.75 mm

Second Zone Width for Far Vision: 0.75 mm

Second Zone Width for Near Vision: 0.75 mm

Third Zone Width for Far Vision: 0.50 mm

Area Ratio in Optical Zone (Portion for Far Vision: Portion for NearVision) 55:45

                  TABLE 16                                                        ______________________________________                                               The Present Invention                                                                       Conventional Design                                             R(mm) X(mm)   Z(mm)   R(mm) X(mm) Z(mm)                                ______________________________________                                        First Zone                                                                             7.70    0.00000 7.69745                                                                             7.70  0.00000                                                                             7.69745                            Second Zone                                                                            7.47    0.00000 7.48045                                                                             7.46  0.00225                                                                             7.46950                            Third Zone                                                                             7.71    0.00000 7.73038                                                                             7.70  0.00338                                                                             7.71915                            Fourth Zone                                                                            7.54    0.00000 7.55981                                                                             7.46  0.02963                                                                             7.48833                            Fifth Zone                                                                             7.76    0.00000 7.80618                                                                             7.70  0.02625                                                                             7.75421                            Improved Amount of                                                                              0.07D                                                       Spherical Aberration                                                          ______________________________________                                    

As can be seen from Table 16, in the portions for far vision, the radiusof curvature is monotonously increased and the Z-coordinate is alsomonotonously increased. On the other hand, in the portions for nearvision, the radius of curvature is monotonously increased and theZ-coordinate is also monotonously increased. In addition, in theportions for both of far vision and near vision, all of theX-coordinates are zero. According to the present invention, thespherical aberration was improved by 0.07D.

(17) Example 17 (see FIG. 18)

First Zone Width for Far Vision: 1.00 mm

First Zone Width for Near Vision: 1.00 mm

Second Zone Width for Far Vision: 0.75 mm

Second Zone Width for Near Vision: 0.50 mm

Third Zone Width for Far Vision: 0.50 mm

Area Ratio in Optical Zone (Portion for Far Vision: Portion for NearVision) 62:38

                  TABLE 17                                                        ______________________________________                                               The Present Invention                                                                       Conventional Design                                             R(mm) X(mm)   Z(mm)   R(mm) X(mm) Z(mm)                                ______________________________________                                        First Zone                                                                             7.70    0.00000 7.69745                                                                             7.70  0.00000                                                                             7.69745                            Second Zone                                                                            7.48    0.00000 7.48382                                                                             7.46  0.00300                                                                             7.46961                            Third Zone                                                                             7.72    0.00000 7.73984                                                                             7.70  0.00600                                                                             7.72196                            Fourth Zone                                                                            7.55    0.00000 7.56801                                                                             7.46  0.03413                                                                             7.48974                            Fifth Zone                                                                             7.76    0.00000 7.80545                                                                             7.70  0.02625                                                                             7.75349                            Improved Amount of                                                                              0.07D                                                       Spherical Aberration                                                          ______________________________________                                    

As can be seen from Table 17, in the portions for far vision, the radiusof curvature is monotonously increased and the Z-coordinate is alsomonotonously increased. On the other hand, in the portions for nearvision, the radius of curvature is monotonously increased and theZ-coordinate is also monotonously increased. In addition, in theportions for both of far vision and near vision, all of theX-coordinates are zero. According to the present invention, thespherical aberration was improved by 0.07D.

Referring to FIGS. 10 through 18, as shown in Examples 9 through 17,according to the present invention, the spherical aberration can besignificantly improved over the whole optical zones in comparison withthe conventional design even if the zone widths of the portions for farvision and near vision are different from each other.

Moreover, since the area ratio of the portions for far vision and nearvision in the optical zones can be freely changed, the user is easy tosee a nearer region for reading or desk work if the user wears a contactlens having a great energy ratio of portions for near vision.

In the aforementioned preferred embodiments, while the portions for bothof far vision and near vision have been plus lenses, the portions forfar vision may be minus lenses and the portions for near visions may beplus lenses. In this case, the variations of the radii of curvature andthe centers of curvature are the same as those of Examples 1 through 8.

Referring to FIGS. 19 and 20, the preferred embodiment of a method forproducing a contact lens, according to the present invention, will bedescribed.

Japanese Patent Laid-Open No. 2-83153 discloses a method for producing acontact lens using an abrasive cloth of a soft material. In thispreferred embodiment, the method disclosed in Japanese Patent Laid-OpenNo. 83153 is applied to a contact lens 1 having a front curve 2 formedby altermately arranging a plurality of curved surfaces for far visionand a plurality of curved surfaces for near vision in the form ofconcentric zones.

In FIG. 19, an abrasive cloth 10 of a soft material is mounted on arotating table 11. The rotating table 11 rotates about an abrasive-clothrotation axis 12 so that the abrasive cloth 10 rotates with the rotatingtable 11. A nozzle 13 is arranged below the rotating table 11. The upperportion of the nozzle 13 is covered with the abrasive cloth 10. Afixture 14 is arranged above the nozzle 13. The fixture 14 rotates abouta fixture rotation axis 15. As enlarged and shown in FIG. 20, thecontact lens 1 is supported on the bottom of the fixture 14 via anadhesive 16 adhered to the base curve 3 during the processing of thecontact lens 1.

In addition, compressed air 18 flows through the nozzle 13 upwards. Apart of the abrasive cloth 10 covering the nozzle 13 is blown upwards bythe fluid pressure of the compressed air passing through the nozzle 13,so that the abrasive cloth 10 is brought into tightly contact with thefront curve 2 of the contact lens 1. As an abrasive, a powder of Al2O3dispersed in water or oil is supplied between the front curve 2 and theabrasive cloth 10. The abrasive cloth 10 comprises, e.g., a commerciallyavailable abrasive cloth lined with polyurethane.

Furthermore, the inner diameter of the nozzle 13 is greater than theouter diameter of the front curve 2 so as to prevent the fluid pressurein the periphery of the front curve 2 from decreasing, so that thepressing force can be uniformly applied to the whole front curve 2.

When the abrasive cloth 10 and the fixture 14 rotate about theabrasive-cloth rotation axis 12 and the fixture rotation axis 15,respectively, the front curve 2 is polished by the relative movementbetween the front curve 2 and the abrasive cloth 10.

Other steps of the method for producing the contact lens 1 are the sameas those of conventional methods.

In this preferred embodiment, the abrasive cloth 10 of a soft materialis brought into tightly contact with the front curve 2 of the contactlens 1 by the compressed air 18, so that the front curve 2 is polishedby the relative movement between the front curve 2 and the abrasivecloth 10. Therefore, the abrasive cloth 10 can be flexibly fitted to theshape of the curved surface of the front curve 2, so that it is possibleto uniformly polish the adjacent curved surfaces F1, F2, . . . for farvision and N1, N2, . . . for near vision.

Referring to FIGS. 21 and 22, the preferred embodiment of a mold forproducing a contact lens, according to the present invention, will bedescribed below.

As shown in FIG. 21, a contact lens mold 20 has a front-curve diesurface 22 having a shape corresponding to that of the front curve 2 ofthe contact lens 1.

The front-curve die surface 22 is formed with die curved-surfaces F1',F2', . . . for far vision and die curved-surfaces N1', N2', . . . fornear vision, which have convexoconcave relationship with the curvedsurfaces F1, F2, . . . for far vision and the curved surfaces N1, N2, .. . for near vision of the contact lens 1, respectively. The shapes ofthe die curved-surfaces F1', F2', . . . for far vision and the diecurved-surfaces N1', N2', . . . for near vision are derived by the raytracing method similar to the first preferred embodiment. In fact, theshapes of the curved surfaces F1, F2, . . . for far vision and thecurved surfaces N1, N2, . . . for near vision of the contact lens 1 arederived by the ray tracing method, and then, the shapes of the curvedsurfaces having convexoconcave relationship with the aforementionedcurved surfaces are derived.

Although a number of curved surfaces F1', F2', . . . for far vision andcurved surfaces N1', N2', . . . for near vision are shown in FIG. 21,the number thereof may be optionally up to about 100.

Referring to FIGS. 22 and 23, a method for producing a contact lensusing the contact lens mold 20 will be described below.

As shown in FIG. 22, a lens material 24 is supplied to the front-curvedie surface 22 which is formed with the curved surfaces F1', F2', . . .for far vision and the curved surfaces N1', N2', . . . for near vision.Then, as shown in FIG. 23, a base-curve die 25 having a base-curve diesurface 23, which has convexoconcave relationship with the base curve 3,is put so as to face the front-curve die surface 22 in place. In thisstate, the lens material 24 is polymerized with ultraviolet or heat.Furthermore, a contact lens of a desired shape is formed in a gapbetween the front-curve die surface 22 and the base-curve die surface23.

Referring to FIGS. 22 and 24, another method for producing a contactlens using the contact lens mold 20 will be described below.

As shown in FIG. 22, a lens material 24 is supplied to the front-curvedie surface 22 which is formed with the curved surfaces F1', F2', . . .for far vision and the curved surfaces N1', N2', . . . for near vision.Then, as shown in FIG. 24, the contact lens mold 20 is rotated. The basecurve 3 is formed by controlling the revolving speed of the contact lensmold 20. Furthermore, the base curve may be formed by cutting by meansof a lathe.

In this preferred embodiment, since the curved surfaces F1', F2', . . .for far vision and the curved surfaces N1', N2', . . . for near vision,which have convexoconcave relationship with the curved surfaces F1, F2,. . . for far vision and the curved surfaces N1, N2, . . . for nearvision of the contact lens 1, are formed on the front-curve die surface22 of the contact lens mold 20, it is possible to produce a contact lenswherein the spherical aberrations of portions for far vision and nearvision are removed using the contact lens mold 20.

While the present invention has been disclosed in terms of the preferredembodiment in order to facilitate better understanding thereof, itshould be appreciated that the invention can be embodied in various wayswithout departing from the principle of the invention. Therefore, theinvention should be understood to include all possible embodiments andmodification to the shown embodiments which can be embodied withoutdeparting from the principle of the invention as set forth in theappended claims.

What is claimed is:
 1. A multifocal contact lens having a lens curveformed by alternately arranging a plurality of curved surfaces for farvision and a plurality of spherically curved surfaces for near vision inthe form of concentric zones,each of said plurality of curved surfacesfor far vision of said lens curve having a center of curvature on anoptical axis and a radius of curvature, which is set so that a ray beingincident on the corresponding curved surface and being parallel to theoptical axis forms an image at a location near a single focal point forfar vision on the optical axis, and each of said plurality of curvedsurfaces for near vision of said lens curve having a center of curvatureon the optical axis and a radius of curvature, which is set so that aray being incident on the corresponding curved surface and beingparallel to the optical axis forms an image at a location near a singlefocal point for near vision on the optical axis.
 2. A multifocal contactlens having a lens curve formed by alternately arranging a plurality ofcurved surfaces for far vision and a plurality of spherically curvedsurfaces for near vision in the form of concentric zones,each of saidplurality of curved surfaces for far vision of said lens curve having acenter of curvature on an optical axis and a radius of curvature, whichis set so that a predetermined principal ray being incident on thecorresponding curved surface and being parallel to the optical axispasses through a location near a single focal point for far vision onthe optical axis, and each of said plurality of curved surfaces for nearvision of said lens curve having a center of curvature on the opticalaxis and a radius of curvature, which is set so that a predeterminedprincipal ray being incident on the corresponding curved surface andbeing parallel to the optical axis passes through a location near asingle focal point for near vision on the optical axis.
 3. Themultifocal contact lens according to claim 2, wherein a ray passingthrough a predetermined location in a zone width of each of the curvedsurfaces for far vision and near vision is selected as saidpredetermined principal ray with respect to all the curved surfaces forfar vision and near vision, which include the curved surface for farvision containing the optical axis and the curved surface for nearvision containing the optical axis.
 4. The multifocal contact lensaccording to claim 3, wherein said predetermined location in the zonewidth of each of all the curved surfaces is the center in the zone widthof each of all the curved surfaces.
 5. The multifocal contact lensaccording to claim 2, wherein said predetermined principal ray is a ray,which corresponds to the optical axis with respect to a curved surfacefor far vision containing the optical axis and a curved surface for nearvision containing the optical axis and which passes through apredetermined location in a zone width of each of other curved surfacesfor far vision and near vision with respect to the other curved surfacesfor far vision and near vision.
 6. The multifocal contact lens accordingto claim 1, wherein each of said curved surfaces for far vision has aradius of curvature which is different from those of other curvedsurfaces for far vision, and each of said curved surfaces for nearvision has a radius of curvature which is different from those of othercurved surfaces for near vision.
 7. The multifocal contact lensaccording to claim 1, wherein said lens curve is a front curve.
 8. Themultifocal contact lens according to claim 1, wherein said zone width ofeach of the curved surfaces for far vision varies in accordance with thedistance between each of the curved surfaces for far vision and theoptical axis, and said zone width of each of the curved surfaces fornear vision varies in accordance with the distance between each of thecurved surfaces for near vision and the optical axis.
 9. The multifocalcontact lens according to claim 1, wherein said zone width of each ofthe curved surfaces for far vision increases in accordance with thedistance between each of the curved surfaces for far vision and theoptical axis, and said zone width of each of the curved surfaces fornear vision increases in accordance with the distance between each ofthe curved surfaces for near vision and the optical axis.
 10. Themultifocal contact lens according to claim 1, wherein said zone width ofeach of the curved surfaces for far vision decreases in accordance withthe distance between each of the curved surfaces for far vision and theoptical axis, and said zone width of each of the curved surfaces fornear vision decreases in accordance with the distance between each ofthe curved surfaces for near vision and the optical axis.
 11. Themultifocal contact lens according to claim 1, wherein said zone width ofeach of the curved surfaces for far vision decreases or increases inaccordance with the distance between each of the curved surfaces for farvision and the optical axis, and said zone width of each of the curvedsurfaces for near vision decreases or increases in accordance with thedistance between each of the curved surfaces for near vision and theoptical axis.
 12. The multifocal contact lens according to claim 1,wherein said zone width of each of the curved surfaces for far vision issubstantially the same as those of other curved surfaces for far vision,and said zone width of each of the curved surfaces for near vision issubstantially the same as those of other curved surfaces for nearvision.
 13. The multifocal contact lens according to claim 1, wherein anenergy ratio of the curved surface for far vision to the curved surfacefor near vision is set in accordance with indoor or outdoor use.
 14. Themultifocal contact lens according to claim 13, wherein said energy ratiois an area ratio of the curved surface for far vision to the curvedsurface for near vision.
 15. The multifocal contact lens according toclaim 13, wherein said energy ratio is a ratio of amount of transmittedlight.
 16. A mold for forming a multifocal contact lens having a lenscurve formed by alternately arranging a plurality of spherically curvedsurfaces for far vision and a plurality of curved surfaces for nearvision in the form of concentric zones,each of said plurality of curvedsurfaces for far vision of the lens curve having a center of curvatureon an optical axis and a radius of curvature, which is set so that a raybeing incident on the corresponding curved surface and being parallel tothe optical axis forms an image at a location near a single focal pointfor far vision on the optical axis, and each of said plurality of curvedsurfaces for near vision of the lens curve having a center of curvatureon the optical axis and a radius of curvature, which is set so that aray being incident on the corresponding curved surface and beingparallel to the optical axis forms an image at a location near a singlefocal point for near vision on the optical axis.
 17. A mold for forminga multifocal contact lens having a lens curve formed by alternatelyarranging a plurality of spherically curved surfaces for far vision anda plurality of curved surfaces for near vision in the form of concentriczones,each of said plurality of curved surfaces for far vision of thelens curve having a center of curvature on an optical axis and a radiusof curvature, which is set so that a predetermined principal ray beingincident on the corresponding curved surface and being parallel to theoptical axis passes through a location near a single focal point for farvision on the optical axis, and each of said plurality of curvedsurfaces for near vision of the lens curve having a center of curvatureon the optical axis and a radius of curvature, which is set so that apredetermined principal ray being incident on the corresponding curvedsurface and being parallel to the optical axis passes through a locationnear a single focal point for near vision on the optical axis.
 18. Amethod for producing a multifocal contact lens having a lens curveformed by alternately arranging a plurality of curved surfaces for farvision and a plurality of curved surfaces for near vision in the form ofconcentric zones, said method comprising the steps of:determining afocal point for far vision and a focal point for near vision on anoptical axis; sequentially setting a proposal for a center of curvatureand a proposal for a radius of curvature, which define each of thecurved surfaces for far vision and near vision, with respect to each ofthe curved surfaces for far vision and near vision in the order of fromthe curved surface for far vision or near vision nearest the opticalaxis toward the curved surface for near vision or far vision apart fromthe optical axis; sequentially changing said proposal for the center ofcurvature and said proposal for the radius of curvature to carry out theray tracing so that a predetermined principal ray being parallel to theoptical axis passes through said focal point for far vision or saidfocal point for near vision; and deciding said center of curvature ofeach of said curved surfaces on the optical axis, and said radius ofcurvature of each of said curved surfaces.
 19. The method for producinga multifocal contact lens according to claim 18, which further comprisesthe steps of:defining a focal point for far vision and a focal point fornear vision on an optical axis; setting a proposal for a center ofcurvature of a first portion for far vision and a proposal for a radiusof curvature of the first portion for far vision, said center ofcurvature and said radius of curvature of the first portion for farvision defining a first curved surface for far vision containing theoptical axis; deriving an intersection point of a circle, which isdescribed about said proposal for the center of curvature of the firstportion for far vision so as to have a radius being the same as saidproposal for the radius of curvature of the first portion for farvision, with a straight line of the first portion for far vision, whichis parallel to the optical axis defining a zone width of said firstcurved surface for far vision; sequentially changing said proposal forthe center of curvature of the first portion for far vision and saidproposal for the radius of curvature of the first portion for far visionto carry out the ray tracing so that a predetermined principal ray,which is incident on said proposal for the first curved surface for farvision and which is parallel to the optical axis, passes through saidfocal point for far vision, to decide said center of curvature of thefirst portion for far vision and said radius of curvature of the firstportion for far vision; deriving an intersection point of a circle,which is described about the decided center of curvature of the firstportion for far vision so as to have said radius of curvature of thefirst portion for far vision, with the straight line of the firstportion for far vision as an intersection point of the first portion forfar vision, and deciding a curved surface extending from a vertex of thelens curve to the intersection point of the first portion for far visionas the first curved surface for far vision; setting a proposal for aradius of curvature of a first portion for near vision defining a firstcurved surface for near vision outwards adjacent to said first curvedsurface for far vision, and deriving an intersection point of a circle,which is described about the intersection point of the first portion forfar vision so as to have a radius being the same as said proposal forthe radius of curvature of the first portion for far vision, with theoptical axis as a proposal for a center of curvature of the firstportion for near vision; deriving an intersection point of a circle,which is described about said proposal for the center of curvature ofthe first portion for near vision so as to have a radius being the sameas said proposal for the radius of curvature of the first portion fornear vision, with a straight line of the first portion for near vision,which defines a zone width of said first curved surface for near visionand which is parallel to the optical axis, to derive a proposal for thefirst curved surface for near vision; sequentially changing saidproposal for the center of curvature of the first portion for nearvision and said proposal for the radius of curvature of the firstportion for near vision, to carry out the ray tracing so that apredetermined principal ray, which is incident on said proposal for thecurved surface of the first portion for near vision and which isparallel to the optical axis, to decide said center of curvature of thefirst portion for near vision and said radius of curvature of the firstportion for near vision; deriving an intersection point of a circle,which is described about the decided center of curvature of the firstportion for near vision so as to have a radius being the same as saidradius of curvature of the first portion for near vision, with thestraight line of the first portion for near vision as an intersectionpoint of the first portion for near vision, and deciding a curvedsurface extending from the intersection point of the first portion forfar vision to the intersection point of the first portion for nearvision as the first curved surface for near vision; setting a proposalfor a radius of curvature of a second portion for far vision defining acurved surface of a second portion for far vision outsides adjacent tothe first curved surface for near vision, and deriving an intersectionpoint of a circle, which is described about said intersection point ofthe first portion for near vision so as to have a radius being the sameas said proposal for the radius of curvature of the second portion forfar vision, with the optical axis as a proposal for a center ofcurvature of the second portion for far vision; deriving an intersectionpoint of a circle, which is described about said proposal for the centerof curvature of the second portion for far vision so as to have a radiusbeing the same as said proposal for the radius of curvature of thesecond portion for far vision, with a straight line of the secondportion for far vision, which defines a zone width of said second curvedsurface for far vision and which is parallel to the optical axis;sequentially changing said proposal for the center of curvature of thesecond portion for far vision and said proposal for the radius ofcurvature of the second portion for far vision, to carry out the raytracing so that a predetermined principal ray, which is incident on saidproposal for the second curved surface for far vision and which isparallel to the optical axis, passes through said focal point for farvision, to decide said center of curvature of the second portion for farvision and said radius of curvature of the second portion for farvision; deriving an intersection point of a circle, which is describedabout the decided center of curvature of the second portion for farvision so as to have a radius being the same radius of curvature of thesecond portion for far vision, with said straight line of the secondportion for far vision as an intersection portion of the second portionfor far vision, and deciding a curved surface extending from saidintersection point of the first portion for near vision to saidintersection point of the second portion for far vision as said secondcurved surface for far vision; setting a proposal for a radius ofcurvature of a second portion for near vision defining a curved surfaceof a second portion for near vision outwards adjacent to said secondcurved surface for far vision, and deciding an intersection point of acircle, which is described about said intersection point of the secondportion for far vision so as to have a radius being the same as saidproposal for the radius of curvature of the second portion for nearvision, with the optical axis as a proposal for a center of curvature ofthe second portion for near vision; deriving an intersection point of acircle, which is described about said proposal for the center ofcurvature of the second portion for near vision so as to have a radiusbeing the same as said proposal for the radius of curvature of thesecond portion for near vision, with a straight line of a second portionfor near vision, which defines a zone width of said second curvedsurface for near vision and which is parallel to the optical axis, toderive a proposal for the second curved surface for near vision;sequentially changing said proposal for the center of curvature of thesecond portion for near vision and said proposal for the radius ofcurvature of the second portion for near vision, to carry out the raytracing so that a predetermined principal ray, which is incident on saidproposal for the second curved surface for near vision and which isparallel to the optical axis, passes through said focal point for nearvision, to decide said center of curvature of the second portion fornear vision and said radius of curvature of the second portion for nearvision; and deriving an intersection point of a circle, which isdescribed about the decided center of curvature of the second portionfor near vision so as to have a radius being the same as said radius ofcurvature of the second portion for near vision, with said straight lineof the second portion for near vision as an intersection point of thesecond portion for near vision, and deciding a curved surface extendingfrom said intersection point of the second portion for far vision tosaid intersection point of the second portion for near vision as saidsecond curved surface for near vision.
 20. The method for producing amultifocal contact lens according to claim 18, which further comprisesthe steps of:defining a focal point for far vision and a focal point fornear vision on an optical axis; setting a proposal for a center ofcurvature of a first portion for near vision and a proposal for a radiusof curvature of a first portion for near vision, said center ofcurvature and said radius of curvature of said first portion for nearvision defining a first curved surface for near vision containing theoptical axis; deriving an intersection point of a circle, which isdescribed about said proposal for the center of curvature of the firstportion for near vision so as to have a radius being the same as saidproposal for the radius of curvature of the first portion for nearvision, with a straight line of the first portion for near vision, whichis parallel to the optical axis defining a zone width of said firstcurved surface for near vision; sequentially changing said proposal forthe center of curvature of the first portion for near vision and saidproposal for the radius of curvature of the first portion for nearvision to carry out the ray tracing so that a predetermined principalray, which is incident on said proposal for the first curved surface fornear vision and which is parallel to the optical axis, passes throughsaid focal point for near vision, to decide said center of curvature ofthe first portion for near vision and said radius of curvature of thefirst portion for near vision; deriving an intersection point of acircle, which is described about the decided center of curvature of thefirst portion for near vision so as to have said radius of curvature ofthe first portion for near vision, with the straight line of the firstportion for near vision as an intersection point of the first portionfor near vision, and deciding a curved surface extending a vertex of thelens curve to the intersection point of the first portion for nearvision as the first curved surface for near vision; setting a proposalfor a radius of curvature of a first portion for far vision defining afirst curved surface for far vision outwards adjacent to said firstcurved surface for near vision, and deriving an intersection point of acircle, which is described about the intersection point of the firstportion for near vision so as to have a radius being the same as saidproposal for the radius of curvature of the first portion for nearvision, with the optical axis as a proposal for a center of curvature ofthe first portion for far vision; deriving an intersection point of acircle, which is described about said proposal for the center ofcurvature of the first portion for far vision so as to have a radiusbeing the same as said proposal for the radius of curvature of the firstportion for far vision, with a straight line of the first portion forfar vision, which defines a zone width of said first curved surface forfar vision and which is parallel to the optical axis, to derive aproposal for the first curved surface for far vision; sequentiallychanging said proposal for the center of curvature of the first portionfor far vision and said proposal for the radius of curvature of thefirst portion for far vision, to carry out the ray tracing so that apredetermined principal ray, which is incident on said proposal for thecurved surface of the first portion for far vision and which is parallelto the optical axis, to decide said center of curvature of the firstportion for far vision and said radius of curvature of the first portionfor far vision; deriving an intersection point of a circle, which isdescribed about the decided center of curvature of the first portion forfar vision so as to have a radius being the same as said radius ofcurvature of the first portion for far vision, with the straight line ofthe first portion for far vision as an intersection point of the firstportion for far vision, and deciding a curved surface extending from theintersection point of the first portion for near vision to theintersection point of the first portion for far vision as the firstcurved surface for far vision; setting a proposal for a radius ofcurvature of a second portion for near vision defining a curved surfaceof a second portion for near vision outsides adjacent to the firstcurved surface for far vision, and deriving an intersection point of acircle, which is described about said intersection point of the firstportion for far vision so as to have a radius being the same as saidproposal for the radius of curvature of the second portion for nearvision, with the optical axis as a proposal for a center of curvature ofthe second portion for near vision; deriving an intersection point of acircle, which is described about said proposal for the center ofcurvature of the second portion for near vision so as to have a radiusbeing the same as said proposal for the radius of curvature of thesecond portion for near vision, with a straight line of the secondportion for near vision, which defines a zone width of said secondcurved surface for near vision and which is parallel to the opticalaxis; sequentially changing said proposal for the center of curvature ofthe second portion for near vision and said proposal for the radius ofcurvature of the second portion for near vision, to carry out the raytracing so that a predetermined principal ray, which is incident on saidproposal for the second curved surface for near vision and which isparallel to the optical axis, passes through said focal point for nearvision, to decide said center of curvature of the second portion fornear vision and said radius of curvature of the second portion for nearvision; deriving an intersection point of a circle, which is describedabout the decided center of curvature of the second portion for nearvision so as to have a radius being the same radius of curvature of thesecond portion for near vision, with said straight line of the secondportion for near vision as an intersection portion of the second portionfor near vision, and deciding a curved surface extending from saidintersection point of the first portion for far vision to saidintersection point of the second portion for near vision as said secondcurved surface for near vision; setting a proposal for a radius ofcurvature of a second portion for far vision defining a second curvedsurface for far vision outwards adjacent to said second curved surfacefor near vision, and deciding an intersection point of a circle, whichis described about said intersection point of the second portion fornear vision so as to have a radius being the same as said proposal forthe radius of curvature of the second portion for far vision, with theoptical axis as a proposal for a center of curvature of the secondportion for far vision; deriving an intersection point of a circle,which is described about said proposal for the center of curvature ofthe second portion for far vision so as to have a radius being the sameas said proposal for the radius of curvature of the second portion forfar vision, with a straight line of the second portion for far vision,which defines a zone width of said second curved surface for far visionand which is parallel to the optical axis, to derive a proposal for thesecond curved surface for far vision; sequentially changing saidproposal for the center of curvature of the second portion for farvision and said proposal for the radius of curvature of the secondportion for far vision, to carry out the ray tracing so that apredetermined principal ray, which is incident on said proposal for thesecond curved surface for far vision and which is parallel to theoptical axis, passes through said focal point for far vision, to decidesaid center of curvature of the second portion for far vision and saidradius of curvature of the second portion for far vision; and derivingan intersection point of a circle, which is described about the decidedcenter of curvature of the second portion for far vision so as to have aradius being the same as said radius of curvature of the second portionfor far vision, with said straight line of the second portion for farvision as an intersection point of the second portion for far vision,and deciding a curved surface extending from said intersection point ofthe second portion for near vision to said intersection point of thesecond portion for far vision as the second curved surface for farvision.
 21. The method for producing a multifocal contact lens accordingto claim 18, wherein said predetermined principal ray is a ray passingthrough a predetermined location in a zone width of each of the curvedsurfaces for far vision and near vision with respect to all the curvedsurfaces for far vision and near vision, which include the curvedsurface for far vision containing the optical axis and the curvedsurface for near vision containing the optical axis.
 22. The method forproducing a multifocal contact lens according to claim 21, wherein saidpredetermined location in the zone width of each of all the curvedsurfaces is the center in the zone width of each of all the curvedsurfaces.
 23. The method for producing multifocal contact lens accordingto claim 18, wherein said predetermined principal ray is a ray, whichcorresponds to the optical axis with respect to a curved surface for farvision containing the optical axis and a curved surface for near visioncontaining the optical axis and which passes through a predeterminedlocation in a zone width of each of curved surfaces with respect toother curved surfaces for far vision and near vision.
 24. The method forproducing multifocal contact lens according to claim 18, wherein each ofsaid curved surfaces for far vision has a radius of curvature which isdifferent from those of other curved surfaces for far vision, and eachof said curved surfaces for near vision has a radius of curvature whichis different from those of other curved surfaces for near vision. 25.The method for producing multifocal contact lens according to claim 18,wherein said lens curve is a front curve.
 26. A method for producing amultifocal contact lens having a lens curve formed by altermatelyarranging a plurality of curved surfaces for far vision and a pluralityof curved surfaces for near vision in the form of concentric zones,eachof said plurality of curved surfaces for far vision of said lens curvehaving a center of curvature on an optical axis and a radius ofcurvature, which is set so that a ray being incident on thecorresponding curved surface and being parallel to the optical axisforms an image at a location near a single focal point for far vision onthe optical axis, and each of said plurality of curved surfaces for nearvision of said lens curve having a center of curvature on the opticalaxis and a radius of curvature, which is set so that a ray beingincident on the corresponding curved surface and being parallel to theoptical axis forms an image at a location near a single focal point fornear vision on the optical axis, said method comprising the stepsof:bringing an abrasive cloth of a soft material into tightly contactwith a front curve of the multifocal contact lens by fluid pressure; andcausing a relative movement between the front curve and the abrasivecloth to polish the front curve.
 27. A method for producing a multifocalcontact lens having a lens curve formed by altermately arranging aplurality of curved surfaces for far vision and a plurality of curvedsurfaces for near vision in the form of concentric zones,each of saidplurality of curved surfaces for far vision of said lens curve having acenter of curvature on an optical axis and a radius of curvature, whichis set so that a predetermined principal ray being incident on thecorresponding curved surface and being parallel to the optical axisforms an image at a location near a single focal point for far vision onthe optical axis, and each of said plurality of curved surfaces for nearvision of said lens curve having a center of curvature on the opticalaxis and a radius of curvature, which is set so that a predeterminedprincipal ray being incident on the corresponding curved surface andbeing parallel to the optical axis forms an image at a location near asingle focal point for near vision on the optical axis, said methodcomprising the steps of:bringing an abrasive cloth of a soft materialinto tightly contact with a front curve of the multifocal contact lensby fluid pressure; and causing a relative movement between the frontcurve and the abrasive cloth to polish the front curve.